submodule--helper: fix a leak in module_add()

Fix a leak in module_path(), since a6226fd (submodule--helper:
convert the bulk of cmd_add() to C, 2021-08-10), we've been freeing
add_data.sm_path, but in this case we clobbered it, and didn't free
the value we clobbered.

This makes test 28 of "t/t7400-submodule-basic.sh" ("submodule add in
subdirectory") pass when we're compiled with SANITIZE=leak..

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Reviewed-by: Glen Choo <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

builtin/submodule--helper.c

@@ -3314,8 +3314,12 @@ static int module_add(int argc, const char **argv, const char *prefix)
 	else
 		add_data.sm_path = xstrdup(argv[1]);
 
-	if (prefix && *prefix && !is_absolute_path(add_data.sm_path))
-		add_data.sm_path = xstrfmt("%s%s", prefix, add_data.sm_path);
+	if (prefix && *prefix && !is_absolute_path(add_data.sm_path)) {
+		char *sm_path = add_data.sm_path;
+
+		add_data.sm_path = xstrfmt("%s%s", prefix, sm_path);
+		free(sm_path);
+	}
 
 	if (starts_with_dot_dot_slash(add_data.repo) ||
 	    starts_with_dot_slash(add_data.repo)) {