mml2015: make key image extraction robuster

* mml2015.el (mml2015-epg-key-image): Use 'gnus-create-image' instead
of 'create-image' for XEmacs compatibility; check errors when decoding
image.  Reported by Uwe Brauer.
gnus-sum.el (gnus-summary-from-or-to-or-newsgroups): Revert 2013-01-14 change