Add some safeguards against NULL base msg
* src/common.h (ERR_GET_BASE_MSG_FAILED): Add new bug code. * src/mailitem-events.cpp (Send): Check for null base msg * src/windowmessages.cpp (gpgol_window_proc): ditto. * src/ribbon-callbacks.cpp (get_mail_from_control): ditto.
Except for get_mail_from_control this should never be hit
(thus the bug error) but accessing a pointer before checking
it seemed wrong.