vc1: check the source buffer in vc1_mc functions

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/vc1dec.c

@@ -407,6 +407,11 @@ static void vc1_mc_1mv(VC1Context *v, int dir)
         use_ic = v->next_use_ic;
     }
 
+    if (!srcY || !srcU) {
+        av_log(v->s.avctx, AV_LOG_ERROR, "Referenced frame missing.\n");
+        return;
+    }
+
     src_x   = s->mb_x * 16 + (mx   >> 2);
     src_y   = s->mb_y * 16 + (my   >> 2);
     uvsrc_x = s->mb_x *  8 + (uvmx >> 2);
@@ -583,6 +588,11 @@ static void vc1_mc_4mv_luma(VC1Context *v, int n, int dir, int avg)
         use_ic = v->next_use_ic;
     }
 
+    if (!srcY) {
+        av_log(v->s.avctx, AV_LOG_ERROR, "Referenced frame missing.\n");
+        return;
+    }
+
     if (v->field_mode) {
         if (v->cur_field_type != v->ref_field_type[dir])
             my = my - 2 + 4 * v->cur_field_type;
@@ -879,6 +889,11 @@ static void vc1_mc_4mv_chroma(VC1Context *v, int dir)
         use_ic = v->next_use_ic;
     }
 
+    if (!srcU) {
+        av_log(v->s.avctx, AV_LOG_ERROR, "Referenced frame missing.\n");
+        return;
+    }
+
     srcU += uvsrc_y * s->uvlinesize + uvsrc_x;
     srcV += uvsrc_y * s->uvlinesize + uvsrc_x;