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

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

Fix a leak in module_path(), since a6226fd772b (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 <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>