Prevent data loss when an ssl socket is closed by remote

Prevent data loss when an ssl socket is closed by remote

SSL context was destroyed on disconnect. This makes it impossible to
decrypt buffered encrypted data. So if there is encrypted data in the
receive buffers, then don't destroy the ssl context until the socket is
destroyed.

Task-Number: QTBUG-23607
Change-Id: I16a7b4fa006647ec73049c90cdbc72686696850f
Reviewed-by: Jonas Gastal <jgastal@profusion.mobi>
Reviewed-by: Richard J. Moore <rich@kde.org>