builtin/submodule--helper: fix leaking remote ref on errors

When `update_submodule()` fails we return with `die_message()`, which
only causes us to print the same message as `die()` would without
actually causing the process to die. We don't free memory in that case
and thus leak memory.

Fix the leak by freeing the remote ref.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7420-submodule-set-url.sh

@@ -10,6 +10,7 @@ as expected.
 '
 
 TEST_NO_CREATE_REPO=1
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 test_expect_success 'setup' '