h264: reset data_partitioning if decoding the slice header for NAL_DPA fails

If it was set before then we can end up trying to decode a slice without
a valid slice header, which can lead to invalid memory access.

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

libavcodec/h264.c

@@ -4725,8 +4725,13 @@ again:
                 hx->intra_gb_ptr =
                 hx->inter_gb_ptr = NULL;
 
-                if ((err = decode_slice_header(hx, h)) < 0)
+                if ((err = decode_slice_header(hx, h)) < 0) {
+                    /* make sure data_partitioning is cleared if it was set
+                     * before, so we don't try decoding a slice without a valid
+                     * slice header later */
+                    h->data_partitioning = 0;
                     break;
+                }
 
                 hx->data_partitioning = 1;
                 break;