demux_null: fix segfault with --cache enabled

stream->info can be NULL if it's the cache wrapper. To be fair,
stream->info is considered private API anyway. So don't access it, but
check the URL instead.

demux/demux_null.c

@@ -21,7 +21,7 @@
 
 static int try_open_file(struct demuxer *demux, enum demux_check check)
 {
-    if (strcmp(demux->stream->info->name, "null") != 0 &&
+    if (!bstr_startswith0(bstr0(demux->filename), "null://") &&
         check != DEMUX_CHECK_REQUEST)
         return -1;
     return 0;