function is assured to parse at most one packet. This makes this function
useful for ASF data packet parsing in a "push-mode" in addition to the
current "pull-mode", and therefore allows for use of these functions in,
for example, the RTSP demuxer (for MS-RTSP support). Tested to give identical
output before and after for regular ASF playback, also see discussion in the
ML thread "[PATCH] asf.c: move packet_time_start=0 statement". Testsuite also
works after the patch, tested by Benoit Fouet.

Originally committed as revision 16108 to svn://svn.ffmpeg.org/ffmpeg/trunk