mjpegdec: check that the coded dc_index and ac_index have a valid associated VLC table.
Removes some disabled dc_index/ac_index checking code that seems to have had
some undocumented issues and should not really be necessary anymore now.
Fixes from issue 1240 the files mjpeg/smclockmjpeg.avi.1.10 and mjpeg/smclockmjpeg.avi.1.171.

Originally committed as revision 19341 to svn://svn.ffmpeg.org/ffmpeg/trunk
