submodule: fix cwd leak in get_superproject_working_tree()

get_superproject_working_tree() allocates cwd via xgetcwd() at
the top of the function, but two early-return paths (when not
inside a work tree, and when strbuf_realpath for "../" fails)
return 0 without freeing it.

Redirect these early returns through a cleanup label that frees
cwd before returning.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

submodule.c

@@ -2626,10 +2626,10 @@ int get_superproject_working_tree(struct strbuf *buf)
 		 * We might have a superproject, but it is harder
 		 * to determine.
 		 */
-		return 0;
+		goto out;
 
 	if (!strbuf_realpath(&one_up, "../", 0))
-		return 0;
+		goto out;
 
 	subpath = relative_path(cwd, one_up.buf, &sb);
 	strbuf_release(&one_up);
@@ -2692,6 +2692,10 @@ int get_superproject_working_tree(struct strbuf *buf)
 		die(_("ls-tree returned unexpected return code %d"), code);
 
 	return ret;
+
+out:
+	free(cwd);
+	return 0;
 }
 
 /*