[Gorm, -gui] fix for #46317

[Graham Lee]

Bug: https://savannah.gnu.org/bugs/?46317

The fix is to always return an image with the correct name, instead of -nil, from -[NSImage initWithCoder:].

Steps to reproduce: open the .gorm file in this project: https://github.com/iamleeg/test-nsimage-name

Changelog entry:

2017-10-28  Graham Lee  <graham@iamleeg.com>

    * Source/NSImage.m (initWithCoder:): restore image name when
    loading an archived image with a name that can't be resolved at
    load time. Fixes Gorm bug #46317.

Patch: https://github.com/gnustep/libs-gui/pull/12 / https://github.com/gnustep/libs-gui/pull/12.patch

Cheers,
Graham.