Fix potential compiler warning, seen under MinGW with gcc 4.8.1.

In fact this warning is a false positive, but fixed anyway.
Warning was: x, y may be used uninitialized (or similar).


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10391 ea41ed52-d2ee-0310-a9c1-e6b18d33e121