This avoids a crash when the next slice is not a start slice and thus
pkt->data is still NULL.
This probably only happens with broken or unsupported files like
http://samples.mplayerhq.hu/real/multirate/JustaSpa1937_64kb.rm
that need further fixes, but keeping vst state consistent is still a good idea.

Originally committed as revision 19830 to svn://svn.ffmpeg.org/ffmpeg/trunk