submodule--helper: fix a configure_added_submodule() leak

Fix config API a memory leak added in a452128a36 (submodule--helper:
introduce add-config subcommand, 2021-08-06) by using the *_tmp()
variant of git_config_get_string().

In this case we're only checking whether
the (repo|git)_config_get_string() call is telling us that the
"submodule.active" key exists.

As with the preceding commit we'll find many other such patterns in
the codebase if we go fishing. E.g. "git gc" leaks in the code added
in 61f7a383d3 (maintenance: use 'incremental' strategy by default,
2020-10-15). Similar code in "git gc" added in
b08ff1fee0 (maintenance: add --schedule option and config,
2020-09-11) doesn't leak, but we could avoid the malloc() & free() in
that case.

A coccinelle rule to find those would find and fix some leaks, and
cases where we're doing needless malloc() + free()'s but only care
about the key existence, or are copying
the (repo|git)_config_get_string() return value right away.

But as with the preceding commit let's punt on all of that for now,
and just narrowly fix this specific case in submodule--helper.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -3157,7 +3157,7 @@ static int config_submodule_in_gitmodules(const char *name, const char *var, con
 static void configure_added_submodule(struct add_data *add_data)
 {
 	char *key;
-	char *val = NULL;
+	const char *val;
 	struct child_process add_submod = CHILD_PROCESS_INIT;
 	struct child_process add_gitmodules = CHILD_PROCESS_INIT;
 
@@ -3202,7 +3202,7 @@ static void configure_added_submodule(struct add_data *add_data)
 	 * is_submodule_active(), since that function needs to find
 	 * out the value of "submodule.active" again anyway.
 	 */
-	if (!git_config_get_string("submodule.active", &val)) {
+	if (!git_config_get_string_tmp("submodule.active", &val)) {
 		/*
 		 * If the submodule being added isn't already covered by the
 		 * current configured pathspec, set the submodule's active flag

t/t7413-submodule-is-active.sh

@@ -9,6 +9,7 @@ This is a unit test of the submodule.c is_submodule_active() function,
 which is also indirectly tested elsewhere.
 '
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 test_expect_success 'setup' '