Fix T60957: ASSERT when reloading double-linked file.

Kind of funny to see that this has been missing presumably since the
first version of library linking in Blender, and only gets noticed now.
Then again, that was not really a critical issue, iirc write code
ensures all libraries directly used get properly written, even if flags
are incorrect.

source/blender/blenkernel/intern/library.c

@@ -168,6 +168,7 @@ void id_lib_extern(ID *id)
 		if (id->tag & LIB_TAG_INDIRECT) {
 			id->tag &= ~LIB_TAG_INDIRECT;
 			id->tag |= LIB_TAG_EXTERN;
+			id->lib->parent = NULL;
 		}
 	}
 }