Mail: fixed duplicate resolving.

Mail: fixed duplicate resolving.

When using SMTP with SSL and resolver, read events might be enabled
during address resolving, leading to duplicate ngx_mail_ssl_handshake_handler()
calls if something arrives from the client, and duplicate session
initialization - including starting another resolving.  This can lead
to a segmentation fault if the session is closed after first resolving
finished.  Fix is to block read events while resolving.

Reported by Robert Norris,
http://mailman.nginx.org/pipermail/nginx/2019-July/058204.html.