Move frame discarding out of the ff_rm_parse_packet() loop, and respect
rm->audio_pkt_cnt in case multiple packets should be read before the next
syncpoint in the file, so that ffplay -an on a file containing AAC audio
works. See "[PATCH] Make RM demuxer behave better with -an option" thread
on mailinglist.

Originally committed as revision 17908 to svn://svn.ffmpeg.org/ffmpeg/trunk
