rtpdec: Don't set RTP timestamps if they already are set by the depacketizer

For MS-RTSP, we don't always get RTCP packets (never?), so the earlier
timestamping code never wrote anything into pkt->pts. The rtpdec_asf
depacketizer just sets the dts of the packet, so if the generic RTP
timestamping is used, too, we get inconsistent timestamps.

Therefore, skip the generic RTP timestamp algorithm if the depacketizer
already has set something.

This fixes "Invalid timestamps" warnings, present since SVN rev 26187.

Originally committed as revision 26241 to svn://svn.ffmpeg.org/ffmpeg/trunk
