h264: do not call field_end if we do not have a current picture

Fixes invalid reads.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org

libavcodec/h264.c

@@ -3348,7 +3348,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
     first_mb_in_slice = get_ue_golomb(&h->gb);
 
     if (first_mb_in_slice == 0) { // FIXME better field boundary detection
-        if (h0->current_slice && FIELD_PICTURE(h)) {
+        if (h0->current_slice && h->cur_pic_ptr && FIELD_PICTURE(h)) {
             field_end(h, 1);
         }