h264: Fix parameters to ff_er_add_slice() call

s->mb_x is reset to zero a couple of lines above. It does not make
sense to call ff_er_add_slice() with 0 as endx when the end of the
macroblock row was reached. Fixes unnecessary and counterproductive
error resilience in https://bugzilla.libav.org/show_bug.cgi?id=394.

CC: libav-stable@libav.org

libavcodec/h264.c

@@ -3567,7 +3567,7 @@ static int decode_slice(struct AVCodecContext *avctx, void *arg)
                         return 0;
                     } else {
                         ff_er_add_slice(s, s->resync_mb_x, s->resync_mb_y,
-                                        s->mb_x, s->mb_y,
+                                        s->mb_x - 1, s->mb_y,
                                         ER_MB_END & part_mask);
 
                         return -1;