rebase: use child_process_clear() to clean

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>