vc1dec: prevent null ptr dereferences.

The added checks are in line with existing checks but should
probably be replaced by more advanced error concealment at some
point.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vc1dec.c

@@ -395,6 +395,9 @@ static void vc1_mc_1mv(VC1Context *v, int dir)
         }
     }
 
+    if(!srcY)
+        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);
@@ -570,6 +573,9 @@ static void vc1_mc_4mv_luma(VC1Context *v, int n, int dir)
     } else
         srcY = s->next_picture.f.data[0];
 
+    if(!srcY)
+        return;
+
     if (v->field_mode) {
         if (v->cur_field_type != v->ref_field_type[dir])
             my = my - 2 + 4 * v->cur_field_type;
@@ -859,6 +865,9 @@ static void vc1_mc_4mv_chroma(VC1Context *v, int dir)
         srcV = s->next_picture.f.data[2];
     }
 
+    if(!srcU)
+        return;
+
     srcU += uvsrc_y * s->uvlinesize + uvsrc_x;
     srcV += uvsrc_y * s->uvlinesize + uvsrc_x;