now checking for NULL window in fgDeactivateMenu (thanks for Ioannis to

point out this occurs)
now dealing with the aftermath of the corner case bugs of different
menus in main and child windows both being open at the same time (which
is a bug in itself, but can't figure out a fix). No more crash at least!

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1359 7f0cb862-5218-0410-a997-914c9d46530a