avcodec/bsf: check that AVBSFInternal was allocated before dereferencing it

This can happen when av_bsf_free() is called on av_bsf_alloc() failure.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/bsf.c

@@ -47,6 +47,7 @@ void av_bsf_free(AVBSFContext **pctx)
 
     av_opt_free(ctx);
 
+    if (ctx->internal)
         av_packet_free(&ctx->internal->buffer_pkt);
     av_freep(&ctx->internal);
     av_freep(&ctx->priv_data);