Cleanup: set `fd->mainlist` to NULL at the end of `blo_read_file_internal()`.

This is a local var, do not try to use it later!

source/blender/blenloader/intern/readfile.c

@@ -8347,6 +8347,8 @@ BlendFileData *blo_read_file_internal(FileData *fd, const char *filepath)
 	
 	link_global(fd, bfd);	/* as last */
 	
+	fd->mainlist = NULL;  /* Safety, this is local variable, shall not be used afterward. */
+
 	return bfd;
 }