builtin/credential-cache: fix trivial leaks

builtin/credential-cache: fix trivial leaks

There are two trivial leaks in git-credential-cache(1):

  - We leak the child process in `spawn_daemon()`. As we do not call
    `finish_command()` and instead let the created process daemonize, we
    have to clear the process manually.

  - We do not free the computed socket path in case it wasn't given via
    `--socket=`.

Plug both of these memory leaks.

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