builtin/submodule--helper: clear child process when not running it

builtin/submodule--helper: clear child process when not running it

In `runcommand_in_submodule_cb()` we may end up not executing the child
command when `argv` is empty. But we still populate the command with
environment variables and other things, which needs cleanup. This leads
to a memory leak because we do not call `finish_command()`.

Fix this by clearing the child process when we don't execute it.

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