tests/avtp: Fix coverity issues
Fixes sign extension issues, as well as unchecked return values.
CID: 1465073, 1465074, 1465077
For CID 1465075 and 1465076, I believe they are bogus: the idea is to print a 32 bit timestamp (avtp timestamps are 32 bits), which will naturally fail the test against uint64 "-1" done by the gstreamer macro. Unless I'm missing anything, of course.