Avoid out of picture accesses when CODEC_FLAG_EMU_EDGE is set.

Fixes issue427.

Originally committed as revision 15418 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -2391,7 +2391,7 @@ static inline void xchg_mb_border(H264Context *h, uint8_t *src_y, uint8_t *src_c
         deblock_top  = h->slice_table[mb_xy] == h->slice_table[h->top_mb_xy];
     } else {
         deblock_left = (s->mb_x > 0);
-        deblock_top =  (s->mb_y > 0);
+        deblock_top =  (s->mb_y > !!MB_FIELD);
     }
 
     src_y  -=   linesize + 1;