rmdec: Use the AVIOContext given as parameter in rm_read_metadata()

This fixes crashes when playing back certain RealRTSP streams.

When invoked from the RTP depacketizer, the full realmedia
demuxer isn't invoked, but only certain functions from it, where
a separate AVIOContext is passed in as parameter (for the buffer
containing the data to parse). The functions called from within
those entry points should only be using that parameter, not
s->pb. In the depacketizer case, s is the RTSP context, where ->pb
is null.

Cc: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>