Enhance JSON response parsing, prevent error leak

If the JSON response from speedtest does not contain a property with the
value - `"type": "result"`, then a problem has occurred and the script
should not attempt to extract the values from the JSON object or perform
a database save.

Currently, a non-user friendly error message is shown when a problem
occurs, but we should prevent this script error and show a more friendly
message.

Another side effect of not handling the error is that the script will
exit with a non-zero status, in effect killing the container.

Network problems may be intermittent, and if the container is running
the speed tests in a loop, we want to be able to handle those
intermittent problems without the container dying.

Before this change:

```sh
docker run -e SPEEDTEST_SERVER_ID=123 -e LOOP=true -e LOOP_DELAY=5 barryevans80/speedtest:wip
Running speedtest forever... ♾️

Running a Speed Test with Server ID 123...
{"type":"log","timestamp":"2024-06-17T13:34:56Z","message":"Configuration - No servers defined (NoServersException)","level":"error"}
./speedtest.sh: 33: arithmetic expression: expecting primary: "  / 125000 "
```

After this change:

```sh
docker run -e SPEEDTEST_SERVER_ID=123 -e LOOP=true -e LOOP_DELAY=5 barryevans80/speedtest:wip
Running speedtest forever... ♾️

Running a Speed Test with Server ID 123...
{"type":"log","timestamp":"2024-06-17T13:33:29Z","message":"Configuration - No servers defined (NoServersException)","level":"error"}
[error] Unable to retrieve JSON results from speedtest, please see previous log message
Running next test in 5s...

Running a Speed Test with Server ID 123...
{"type":"log","timestamp":"2024-06-17T13:33:36Z","message":"Configuration - No servers defined (NoServersException)","level":"error"}
[error] Unable to retrieve JSON results from speedtest, please see previous log message
Running next test in 5s...
```

---

The validation required to make the script more robust is achieved by
using a simple jq `select` function:
- `echo "$JSON" | jq 'select(.type == "result")'`

See more about the jq `select` function here:
- https://jqlang.github.io/jq/manual/#select

---

Some shellcheck fixes have been applied:
1. `$/${} is unnecessary on arithmetic variables.` - https://github.com/koalaman/shellcheck/wiki/SC2004
2. `Double quote to prevent globbing and word splitting.` - https://github.com/koalaman/shellcheck/wiki/SC2086

---

Sample successful json response
```json
{
  "type": "result",
  "timestamp": "2024-06-17T05:27:00Z",
  "ping": {
    "jitter": 0.852,
    "latency": 288.814,
    "low": 287.153,
    "high": 289.37
  },
  "download": {
    "bandwidth": 1591349,
    "bytes": 22220160,
    "elapsed": 15010,
    "latency": {
      "iqm": 286.068,
      "low": 282.946,
      "high": 293.54,
      "jitter": 1.568
    }
  },
  "upload": {
    "bandwidth": 38988010,
    "bytes": 493973670,
    "elapsed": 15005,
    "latency": {
      "iqm": 295.051,
      "low": 286.54,
      "high": 379.607,
      "jitter": 7.626
    }
  },
  ...
}
```

Sample failed json response
```json
{
  "type": "log",
  "timestamp": "2024-06-17T05:25:48Z",
  "message": "Configuration - No servers defined (NoServersException)",
  "level": "error"
}
```

speedtest.sh

@@ -27,24 +27,30 @@ run_speedtest()
       JSON=$(speedtest --accept-license --accept-gdpr -f json)
     fi
 
-    DOWNLOAD="$(echo $JSON | jq -r '.download.bandwidth')"
-    UPLOAD="$(echo $JSON | jq -r '.upload.bandwidth')"
-    PING="$(echo $JSON | jq -r '.ping.latency')"
-    echo "Your download speed is $(($DOWNLOAD  / 125000 )) Mbps ($DOWNLOAD Bytes/s)."
-    echo "Your upload speed is $(($UPLOAD  / 125000 )) Mbps ($UPLOAD Bytes/s)."
-    echo "Your ping is $PING ms."
+    json_result_found="$(echo "$JSON" | jq 'select(.type == "result")')"
 
-    # Save results in the database
-    if $DB_SAVE; 
-    then
-        echo "Saving values to database..."
-        curl -s -S -XPOST "$DB_HOST/write?db=$DB_NAME&precision=s&u=$DB_USERNAME&p=$DB_PASSWORD" \
-            --data-binary "download,host=$HOSTNAME value=$DOWNLOAD $DATE"
-        curl -s -S -XPOST "$DB_HOST/write?db=$DB_NAME&precision=s&u=$DB_USERNAME&p=$DB_PASSWORD" \
-            --data-binary "upload,host=$HOSTNAME value=$UPLOAD $DATE"
-        curl -s -S -XPOST "$DB_HOST/write?db=$DB_NAME&precision=s&u=$DB_USERNAME&p=$DB_PASSWORD" \
-            --data-binary "ping,host=$HOSTNAME value=$PING $DATE"
-        echo "Values saved."
+    if [ -n "${json_result_found}" ]; then
+      DOWNLOAD="$(echo "$JSON" | jq -r '.download.bandwidth')"
+      UPLOAD="$(echo "$JSON" | jq -r '.upload.bandwidth')"
+      PING="$(echo "$JSON" | jq -r '.ping.latency')"
+      echo "Your download speed is $((DOWNLOAD  / 125000 )) Mbps ($DOWNLOAD Bytes/s)."
+      echo "Your upload speed is $((UPLOAD  / 125000 )) Mbps ($UPLOAD Bytes/s)."
+      echo "Your ping is $PING ms."
+
+      # Save results in the database
+      if $DB_SAVE;
+      then
+          echo "Saving values to database..."
+          curl -s -S -XPOST "$DB_HOST/write?db=$DB_NAME&precision=s&u=$DB_USERNAME&p=$DB_PASSWORD" \
+              --data-binary "download,host=$HOSTNAME value=$DOWNLOAD $DATE"
+          curl -s -S -XPOST "$DB_HOST/write?db=$DB_NAME&precision=s&u=$DB_USERNAME&p=$DB_PASSWORD" \
+              --data-binary "upload,host=$HOSTNAME value=$UPLOAD $DATE"
+          curl -s -S -XPOST "$DB_HOST/write?db=$DB_NAME&precision=s&u=$DB_USERNAME&p=$DB_PASSWORD" \
+              --data-binary "ping,host=$HOSTNAME value=$PING $DATE"
+          echo "Values saved."
+      fi
+    else
+      echo "[error] Unable to retrieve JSON results from speedtest, please see previous log message"
     fi
 }