Don't access RMContext directly in rdt.c. Rather, use the return value of

ff_rm_parse_packet() to indicate whether more audio packets are available
in the demuxer from the last RM frame, and save that in the RDT parsing
context. See patch/discussion in "[PATCH] rdt.c: don't access RMContext"
on ML.

Originally committed as revision 16110 to svn://svn.ffmpeg.org/ffmpeg/trunk