Add more correct bound checking in filter_mb().

Should fix decoding of CVMAPAQP3_Sony_E.jsv in MinGW

Originally committed as revision 17806 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -6493,7 +6493,7 @@ static void filter_mb( H264Context *h, int mb_x, int mb_y, uint8_t *img_y, uint8
         int qp = s->current_picture.qscale_table[mb_xy];
         if(qp <= qp_thresh
            && (mb_x == 0 || ((qp + s->current_picture.qscale_table[mb_xy-1] + 1)>>1) <= qp_thresh)
-           && (mb_y == 0 || ((qp + s->current_picture.qscale_table[h->top_mb_xy] + 1)>>1) <= qp_thresh)){
+           && (h->top_mb_xy < 0 || ((qp + s->current_picture.qscale_table[h->top_mb_xy] + 1)>>1) <= qp_thresh)){
             return;
         }
     }