Workaround crash when accessing CurrentView

* src/explorer-events.cpp (EVENT_SINK_INVOKE),
src/ribbon-callbacks.cpp (get_mail_from_control): Use
preview pane instead.

Somehow latest Outlook 2016 crashes when accessing the current view
of the Explorer. This is even reproducible with
GpgOL disabled and only with Outlook Spy active. If you select
the explorer of an Outlook.com resource and then access
the CurrentView and close the CurrentView again in Outlook Spy
outlook crashes.

• GnuPG-Bug-Id: T3484