Fix logic error causing panic storing nil

[johnmaguire]

While writing a test I received the following error:

time="2024-06-18T23:14:56-04:00" level=info msg="update failed" error="failed to call defined networking: failed to make API call to Defined Networking: dnclient endpoint returned bad status code '500', body: unexpected request\n"
panic: sync/atomic: store of nil value into Value

goroutine 113 [running]:
sync/atomic.(*Value).Store(0x0?, {0x0, 0x0})
	/opt/homebrew/Cellar/go/1.22.4/libexec/src/sync/atomic/value.go:49 +0xec
github.com/DefinedNet/dnapi.(*Client).streamingPostDNClient.func1()
	/Users/jmaguire/go/pkg/mod/github.com/!defined!net/dnapi@v0.0.0-20240606181337-15680178c26a/client.go:353 +0x314
created by github.com/DefinedNet/dnapi.(*Client).streamingPostDNClient in goroutine 101
	/Users/jmaguire/go/pkg/mod/github.com/!defined!net/dnapi@v0.0.0-20240606181337-15680178c26a/client.go:326 +0x310
FAIL	github.com/DefinedNet/dnclient	0.212s

Basically, we try to decode a "derp" error when receiving a 500. If we fail to decode it, we store a "generic" error, otherwise we store the fancy typed error.

Except I forgot an "else" so even in error, we'd try to overwrite the generic error with a nil error. It's kind of lucky that atomics don't allow storing nil values, or we might've missed this.