x11/fbdesk: Fix build with Clang 6

Image.cc:88:16: error: cannot initialize return object of type 'FbTk::PixmapWithMask *' with an rvalue of type 'bool'

return false;
       ^~~~~

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/fbdesk-1.4.1_6.log