Check mb_pos is big enough before trying to access data left or above.

Fixes out-of-bound access for http://samples.mplayerhq.hu/V-codecs/WMV9/nokia_n90.wmv

Originally committed as revision 7324 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vc1.c

@@ -3072,8 +3072,8 @@ static int vc1_decode_intra_block(VC1Context *v, DCTELEM block[64], int n, int c
         ac_val -= 16 * s->block_wrap[n];
 
     q1 = s->current_picture.qscale_table[mb_pos];
-    if(dc_pred_dir && c_avail) q2 = s->current_picture.qscale_table[mb_pos - 1];
-    if(!dc_pred_dir && a_avail) q2 = s->current_picture.qscale_table[mb_pos - s->mb_stride];
+    if(dc_pred_dir && c_avail && mb_pos) q2 = s->current_picture.qscale_table[mb_pos - 1];
+    if(!dc_pred_dir && a_avail && mb_pos >= s->mb_stride) q2 = s->current_picture.qscale_table[mb_pos - s->mb_stride];
     if(n && n<4) q2 = q1;
 
     if(coded) {