check return value of decode_cabac_mb_dqp, it is INT_MIN on error.
fixes crash with http://stud4.tuwien.ac.at/~e0326863/ats/bbc-sample_small.mpg
and MPlayer's broken packetizer (but will not decode right of course).

Originally committed as revision 4595 to svn://svn.ffmpeg.org/ffmpeg/trunk
