QSslSocket (OpenSSL) - handle abort/close on sslErrors emitted

If a user's code, attached to sslErrors signal, calls abort/close
or disconnectFromHost but our SSL socket was configured not to verify a peer,
no need to continue handshake after calling checkSslErrors
(and finally crashing on invalid 'ssl' pointer).

Task-number: QTBUG-53906
Change-Id: I7f185511d278f9d6f16e7d6c5ba424707141459c
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>