fuzz: some enhancements (#1827)

Load some custom configuration (like in the unit tests) and factorize some
(fuzzing) common code.

There is no way to pass file paths to the fuzzers as parameters. The safe
solution seems to be to load them from the process working dir. Anyway,
missing file is not a blocking error.

Remove some dead code (found looking at the coverage report)

src/lib/protocols/rtp.c

@@ -114,6 +114,7 @@ static u_int8_t isZoom(u_int16_t sport, u_int16_t dport,
 		       u_int16_t *payload_offset) {
   u_int16_t header_offset = sizeof(struct zoom_sfu_encapsulation) + sizeof(struct zoom_media_encapsulation);
 
+  *payload_offset = 0;
   if(payloadLen < header_offset)
     return(0);