Ignore reads when shutdown was triggered

* src/gpgoladdin.cpp (GpgolAddin::shutdown): Set state
variable at begin of shutdown.
* src/mailitem-events.cpp (BeforeRead): Ignore when
shutting down.

This happened when we are closing and closing all mails
but there are multiple inspectors around e.g. if the mail
is open in Outlook Spy. In that case Outlook tries to read
it again immediately after close.