imap-send: free all_msgs strbuf in "out" label

imap-send: free all_msgs strbuf in "out" label

We read stdin into a strbuf, but most code paths never release it,
causing a leak (albeit a minor one, as we leak only when exiting from
the main function of the program).

Commit 56f4f4a29d (imap-send: minimum leakfix, 2024-06-04) did the
minimum to plug the one instance we see in the test suite, when we read
an empty input. But it was sufficient only because aside from this noop
invocation, we don't test imap-send at all!

The right spot to free is in the "out" label, which is hit by all code
paths before leaving the function. We couldn't do that in 56f4f4a29d
because there was no unified exit path. That came separately in
3aca5f7fb0 (imap-send: fix leaking memory in `imap_server_conf`,
2024-06-04), which cleaned up many other leaks (but not this one).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>