h264: dont mess with frame gaps on second fields.

Fixes assertion failure

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

libavcodec/h264.c

@@ -2834,7 +2834,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
             }
         }
 
-        while (h->frame_num != h->prev_frame_num && h->prev_frame_num >= 0 &&
+        while (h->frame_num != h->prev_frame_num && h->prev_frame_num >= 0 && !s0->first_field &&
                h->frame_num != (h->prev_frame_num + 1) % (1 << h->sps.log2_max_frame_num)) {
             Picture *prev = h->short_ref_count ? h->short_ref[0] : NULL;
             av_log(h->s.avctx, AV_LOG_DEBUG, "Frame num gap %d %d\n",