h264: Fail on DPC its not fully implemented

Fixes part of Ticket1369

Found-by: ami_stuff
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 7cb8663362)

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

libavcodec/h264.c

@@ -4399,6 +4399,9 @@ again:
                 init_get_bits(&hx->inter_gb, ptr, bit_length);
                 hx->inter_gb_ptr = &hx->inter_gb;
 
+                av_log(h->s.avctx, AV_LOG_ERROR, "Partitioned H.264 support is incomplete\n");
+                return AVERROR_PATCHWELCOME;
+
                 if (hx->redundant_pic_count == 0 &&
                     hx->intra_gb_ptr &&
                     hx->s.data_partitioning &&