h264: reset first_field when current_picture_ptr is reset

Fixes NULL pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -4287,6 +4287,7 @@ static int decode_frame(AVCodecContext *avctx, void *data,
  out:
 
         s->current_picture_ptr = NULL;
+        s->first_field = 0;
 
         // FIXME factorize this with the output code below
         out     = h->delayed_pic[0];