Fix crash when looking at msgs without content type

* src/mapihelp.cpp (change_message_class_ipm_note): Check
for ct before using it in string comparison.

The codepath is reached for temporary, discarded mails, e.g.
when a Reply All in the Inline editor is canceled through
Escape.

This fixes: dc48589b3d429d7d156c75b4e7bc784b140f40ce

• GnuPG-Bug-Id: T3960