avcodec/h264_slice: do not skip 2nd field if first was not skipped

Found-by: John Högberg <john.hogberg@ericsson.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_slice.c

@@ -1212,14 +1212,16 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
         return AVERROR_INVALIDDATA;
     }
 
-    if (
-        (h->avctx->skip_frame >= AVDISCARD_NONREF && !h->nal_ref_idc) ||
-        (h->avctx->skip_frame >= AVDISCARD_BIDIR  && sl->slice_type_nos == AV_PICTURE_TYPE_B) ||
-        (h->avctx->skip_frame >= AVDISCARD_NONINTRA && sl->slice_type_nos != AV_PICTURE_TYPE_I) ||
-        (h->avctx->skip_frame >= AVDISCARD_NONKEY && h->nal_unit_type != NAL_IDR_SLICE) ||
-         h->avctx->skip_frame >= AVDISCARD_ALL) {
-         return SLICE_SKIPED;
-     }
+    if (h->current_slice == 0 && !h->first_field) {
+        if (
+            (h->avctx->skip_frame >= AVDISCARD_NONREF && !h->nal_ref_idc) ||
+            (h->avctx->skip_frame >= AVDISCARD_BIDIR  && sl->slice_type_nos == AV_PICTURE_TYPE_B) ||
+            (h->avctx->skip_frame >= AVDISCARD_NONINTRA && sl->slice_type_nos != AV_PICTURE_TYPE_I) ||
+            (h->avctx->skip_frame >= AVDISCARD_NONKEY && h->nal_unit_type != NAL_IDR_SLICE) ||
+            h->avctx->skip_frame >= AVDISCARD_ALL) {
+            return SLICE_SKIPED;
+        }
+    }
 
     // to make a few old functions happy, it's wrong though
     if (!h->setup_finished)