imap-send: explicitly verify the peer certificate

imap-send: explicitly verify the peer certificate

It is a bug to obtain the peer certificate without verifying it.

Having said that, from my reading of
https://www.openssl.org/docs/man1.1.1/man3/SSL_set_verify.html, it would
appear that Git is saved by the fact that it calls
`SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER, NULL)` already early on.

In other words, that `SSL_VERIFY_PEER` combined with the `NULL`
parameter (i.e. no overridden callback) would _already_ verify the peer
certificate.  The fact that we later call `SSL_get_peer_certificate()`
is mistaken by CodeQL to mean that that peer certificate still needs to
be verified, but that had already happened at that point.

Nevertheless, it is better to verify the peer certificate explicitly
than to rely on some side effect that is really hard to reason about
(and that took me more than one business day to analyze fully). It also
makes it easier for static analyzers to validate the correctness of the
code.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>