avformat/utils: Ensure that AVFMT_FLAG_CUSTOM_IO is set before use

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit ba631b79)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -537,6 +537,9 @@ int avformat_open_input(AVFormatContext **ps, const char *filename,
     if (options)
         av_dict_copy(&tmp, *options, 0);
 
+    if (s->pb) // must be before any goto fail
+        s->flags |= AVFMT_FLAG_CUSTOM_IO;
+
     if ((ret = av_opt_set_dict(s, &tmp)) < 0)
         goto fail;