jpegdec: Fix vlc table check for progressive jpegs.

Fixes Ticket834

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mjpegdec.c

@@ -1073,7 +1073,7 @@ int ff_mjpeg_decode_sos(MJpegDecodeContext *s,
         if (s->dc_index[i] <  0 || s->ac_index[i] < 0 ||
             s->dc_index[i] >= 4 || s->ac_index[i] >= 4)
             goto out_of_range;
-        if (!s->vlcs[0][s->dc_index[i]].table || !s->vlcs[1][s->ac_index[i]].table)
+        if (!s->vlcs[0][s->dc_index[i]].table || !(s->progressive ? s->vlcs[2][s->ac_index[0]].table : s->vlcs[1][s->ac_index[i]].table))
             goto out_of_range;
     }