rebase: use child_process_clear() to clean

In the run_am() function, we set up a child_process struct to run "git-am", allocating memory for its args and env strvecs. These are normally cleaned up when we call run_command(). But if we encounter certain errors, we exit the function early and try to clean up ourselves by clearing the am.args field. This leaks the "env" strvec. We should use child_process_clear() instead, which covers both. And more importantly, it future proofs us against the struct ever growing more allocated fields. These are unlikely errors to happen in practice, so they don't actually trigger the leak sanitizer in the tests. But we can add a new test which does exercise one of the paths (and fails SANITIZE=leak without this patch). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>