fix slices when code=0x18, patch by Michael Niedermayer <michael@mplayer.dev.hu>

Originally committed as revision 246 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/msmpeg4.c

@@ -675,6 +675,16 @@ int msmpeg4_decode_mb(MpegEncContext *s,
                     1024, s->mb_width);
             memsetw(&s->dc_val[2][(1) + (s->mb_y) * wrap], 
                     1024, s->mb_width);
+            
+            /* reset AC pred (set previous line to 0) */
+            wrap = s->mb_width * 2 + 2;
+            memsetw(s->ac_val[0][0] + (1 + (2 * s->mb_y) * wrap)*16,
+                    0, 2 * s->mb_width*16);
+            wrap = s->mb_width + 2;
+            memsetw(s->ac_val[1][0] + (1 + (s->mb_y) * wrap)*16,
+                    0, s->mb_width*16);
+            memsetw(s->ac_val[2][0] + (1 + (s->mb_y) * wrap)*16,
+                    0, s->mb_width*16);
 
             s->first_slice_line = 1;
         } else {