vc1dec: prevent a crash due missing pred_flag parameter

Handle pred_flag parameter not given to get_mvdata_interlaced()
Signed-off-by: Vladimir Pantelic <vladoman@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/vc1dec.c

@@ -1133,8 +1133,12 @@ static av_always_inline void get_mvdata_interlaced(VC1Context *v, int *dmv_x,
         *dmv_x = get_bits(gb, v->k_x);
         *dmv_y = get_bits(gb, v->k_y);
         if (v->numref) {
-            *pred_flag = *dmv_y & 1;
-            *dmv_y     = (*dmv_y + *pred_flag) >> 1;
+            if (pred_flag) {
+                *pred_flag = *dmv_y & 1;
+                *dmv_y     = (*dmv_y + *pred_flag) >> 1;
+            } else {
+                *dmv_y     = (*dmv_y + (*dmv_y & 1)) >> 1;
+            }
         }
     }
     else {
@@ -1160,7 +1164,7 @@ static av_always_inline void get_mvdata_interlaced(VC1Context *v, int *dmv_x,
             *dmv_y = (sign ^ ((val >> 1) + offs_tab[index1 >> v->numref])) - sign;
         } else
             *dmv_y = 0;
-        if (v->numref)
+        if (v->numref && pred_flag)
             *pred_flag = index1 & 1;
     }
 }