submodule--helper: fix bad config API usage

Fix bad config API usage added in a452128a36 (submodule--helper:
introduce add-config subcommand, 2021-08-06). After
git_config_get_string() returns successfully we know the "char **dest"
will be non-NULL.

A coccinelle patch that transforms this turns up a couple of other
such issues, one in fetch-pack.c, and another in upload-pack.c:

	@@
	identifier F =~ "^(repo|git)_config_get_string(_tmp)?$";
	identifier V;
	@@
	  !F(..., &V)
	- && (V)

But let's focus narrowly on submodule--helper for now, we can fix
those some other time.

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

@@ -3107,7 +3107,7 @@ 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) && val) {
+	if (!git_config_get_string("submodule.active", &val)) {
 		/*
 		 * If the submodule being added isn't already covered by the
 		 * current configured pathspec, set the submodule's active flag