codec.c: Defensively check the returned samples.

Earlier versions of the codec_opus samples_count callback can return
negative error values on undecodable frames.  This resulted in a divide by
zero exception.

* Added a defensive check in ast_codec_samples_count() for a "negative"
samples count return value.  Log the event and set the count to zero.

ASTERISK-27194

Change-Id: Icf69350307ecbbc80a3d74de46af9bd80ea17819