h264: reset data partitioning at the beginning of each decode call

Prevents using GetBitContexts with data from previous calls.

Fixes access to freed memory.

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

libavcodec/h264.c

@@ -1764,7 +1764,6 @@ static int decode_update_thread_context(AVCodecContext *dst,
     h->picture_structure    = h1->picture_structure;
     h->qscale               = h1->qscale;
     h->droppable            = h1->droppable;
-    h->data_partitioning    = h1->data_partitioning;
     h->low_delay            = h1->low_delay;
 
     for (i = 0; i < MAX_PICTURE_COUNT; i++) {
@@ -4745,6 +4744,13 @@ again:
                 }
                 break;
             case NAL_DPA:
+                if (h->avctx->flags & CODEC_FLAG2_CHUNKS) {
+                    av_log(h->avctx, AV_LOG_ERROR,
+                           "Decoding in chunks is not supported for "
+                           "partitioned slices.\n");
+                    return AVERROR(ENOSYS);
+                }
+
                 init_get_bits(&hx->gb, ptr, bit_length);
                 hx->intra_gb_ptr =
                 hx->inter_gb_ptr = NULL;
@@ -4894,6 +4900,9 @@ static int h264_decode_frame(AVCodecContext *avctx, void *data,
     int ret;
 
     h->flags = avctx->flags;
+    /* reset data partitioning here, to ensure GetBitContexts from previous
+     * packets do not get used. */
+    h->data_partitioning = 0;
 
     /* end of stream, output what is still in the buffers */
 out: