builtin/submodule--helper: fix leak when remote_submodule_branch() failed

builtin/submodule--helper: fix leak when remote_submodule_branch() failed

In builtin/submodule--helper.c:update_submodule(), the variable
remote_name is allocated in get_default_remote_submodule() but
may be leaked if remote_submodule_branch() fails. Although it is
unlikely that remote_submodule_branch() would fail after successfully
obtaining a remote ref name from get_default_remote_submodule(),
it is still possible. To prevent a potential memory leak, add a
call to free(remote_name) at the early exit point.

Signed-off-by: Lidong Yan <502024330056@smail.nju.edu.cn>
Signed-off-by: Junio C Hamano <gitster@pobox.com>