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.