Missing RTP Info should be non-fatal

When using the RTSP method, zoneminder listens for the camera to send the RTP sequence and rpttime values during the initial handshake.  This allows zoneminder to verify that the first video frames are in the proper sequence.  However, some devices just don't send this information (yet they still work fine with ffmpeg & vlc), and all is not lost if we don't have it.

By setting seq & rpttime to zero, we guarantee that the first frame will be flagged as out-of-sequence, but that also triggers the re-sync logic to get back into the proper sequence.

The end user will get warnings in the log similar to the following, but as long as zoneminder understands the stream coming from the camera, it should work.

```
2014-11-29 14:41:53.599501	zmc_m7	22318	WAR	Discarding incomplete frame 0, 0 bytes	/home/abauer/rpmbuild/BUILD/ZoneMinder-1.28digest/src/zm_rtp_source.cpp	336
2014-11-29 14:41:53.494540	zmc_m7	22318	WAR	Discarding frame 0	/home/abauer/rpmbuild/BUILD/ZoneMinder-1.28digest/src/zm_rtp_source.cpp	349
2014-11-29 14:41:53.492548	zmc_m7	22318	WAR	Sequence in probation 2, out of sequence	/home/abauer/rpmbuild/BUILD/ZoneMinder-1.28digest/src/zm_rtp_source.cpp	112
```