h264: Only apply error concealment if theres a frame

Without any correctly decoded slices, there can be no frame.

Fixes out of array reads

Found-by: Rafaël Carré
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -2360,7 +2360,7 @@ static int field_end(H264Context *h, int in_setup)
      * past end by one (callers fault) and resync_mb_y != 0
      * causes problems for the first MB line, too.
      */
-    if (!FIELD_PICTURE)
+    if (!FIELD_PICTURE && h->current_slice)
         ff_er_frame_end(s);
 
     ff_MPV_frame_end(s);