If cleanup_set() gets passed a NULL fnametmp or fname, set

cleanup_fname to NULL.

cleanup.c

@@ -166,7 +166,7 @@ void cleanup_disable(void)
 void cleanup_set(char *fnametmp, char *fname, struct file_struct *file,
 		 int fd_r, int fd_w)
 {
-	cleanup_fname = fnametmp;
+	cleanup_fname = fname ? fnametmp : NULL;
 	cleanup_new_fname = fname;
 	cleanup_file = file;
 	cleanup_fd_r = fd_r;