Fix bug introduced fixing T6646 on April 9

* ribbon-callback.cpp: get_mail_from_control fix accesing an
empty vector

Calling front() on a vector where empty() is true is
undefined. Code rearanged to prevent that.

• Fixes-commit: a95d340e478f8fb9529b49b0518a0fafa4d00bfe
• GnuPG-bug-id: T6646