lavc: fix ff_alloc_packet()

Regression introduced in 740b9ff4.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/utils.c

@@ -1017,6 +1017,7 @@ int ff_alloc_packet2(AVCodecContext *avctx, AVPacket *avpkt, int size)
         return AVERROR(EINVAL);
     }
 
+    if (avctx) {
     av_assert0(!avpkt->data || avpkt->data != avctx->internal->byte_buffer);
     if (!avpkt->data || avpkt->size < size) {
         av_fast_padded_malloc(&avctx->internal->byte_buffer, &avctx->internal->byte_buffer_size, size);
@@ -1024,6 +1025,7 @@ int ff_alloc_packet2(AVCodecContext *avctx, AVPacket *avpkt, int size)
         avpkt->size = avctx->internal->byte_buffer_size;
         avpkt->destruct = NULL;
     }
+    }
 
     if (avpkt->data) {
         void *destruct = avpkt->destruct;