vc1dec: Override invalid macroblock quantizer

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Kostya Shishkov <kostya.shishkov@gmail.com>

libavcodec/vc1dec.c

@@ -1048,6 +1048,11 @@ static void vc1_mc_4mv_chroma4(VC1Context *v)
             mquant = v->altpq;                                 \
         if ((edges&8) && s->mb_y == (s->mb_height - 1))        \
             mquant = v->altpq;                                 \
+        if (!mquant || mquant > 31) {                          \
+            av_log(v->s.avctx, AV_LOG_ERROR,                   \
+                   "Overriding invalid mquant %d\n", mquant);  \
+            mquant = 1;                                        \
+        }                                                      \
     }
 
 /**