Separate the packet fetching from the data reading, so that the data reading

Separate the packet fetching from the data reading, so that the data reading
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