Do not eagerly call next_uNN in fill_bytes with empty remainder

Fix a bug where random numbers are consumed from the Mersenne Twister
even when they will never be used because the remainder from the chunks
iterator in `fill_bytes` is non-empty.

This regression was introduced in:

- #116

Add a basic smoke check for reproducibility with MRI's RNG pulled from
`spinoso-random` in artichoke/artichoke.

src/mt.rs

@@ -310,8 +310,11 @@ impl Mt19937GenRand32 {
             next.copy_from_slice(&chunk);
         }
 
-        dest_chunks
-            .into_remainder()
+        let remainder = dest_chunks.into_remainder();
+        if remainder.is_empty() {
+            return;
+        }
+        remainder
             .iter_mut()
             .zip(self.next_u32().to_le_bytes().iter())
             .for_each(|(cell, &byte)| {

src/mt64.rs

@@ -293,8 +293,11 @@ impl Mt19937GenRand64 {
             next.copy_from_slice(&chunk);
         }
 
-        dest_chunks
-            .into_remainder()
+        let remainder = dest_chunks.into_remainder();
+        if remainder.is_empty() {
+            return;
+        }
+        remainder
             .iter_mut()
             .zip(self.next_u64().to_le_bytes().iter())
             .for_each(|(cell, &byte)| {

tests/ruby_reproducibility.rs

@@ -0,0 +1,32 @@
+use rand_mt::Mt;
+
+// ```ruby
+// # Should double check this is official spec
+// it "returns the same numeric output for a given seed across all implementations and platforms" do
+//   rnd = Random.new(33)
+//   rnd.bytes(2).should == "\x14\\"
+//   rnd.bytes(1000) # skip some
+//   rnd.bytes(2).should == "\xA1p"
+// end
+//
+// it "returns the same numeric output for a given huge seed across all implementations and platforms" do
+//   rnd = Random.new(bignum_value ** 4)
+//   rnd.bytes(2).should == "_\x91"
+//   rnd.bytes(1000) # skip some
+//   rnd.bytes(2).should == "\x17\x12"
+// end
+// ```
+#[test]
+fn spec_bytes() {
+    let mut rng = Mt::new(33);
+    let mut buf = [0; 2];
+    rng.fill_bytes(&mut buf);
+    assert_eq!(buf[..], b"\x14\\"[..]);
+
+    let mut skip = [0; 1000];
+    rng.fill_bytes(&mut skip);
+
+    let mut buf = [0; 2];
+    rng.fill_bytes(&mut buf);
+    assert_eq!(buf[..], b"\xA1p"[..]);
+}