glutCreateMenu can be created with a NULL callback

[rcmaniac25]

As discussed in email thread from 11/18/2015, subject "Possible Bugs"

"glutCreateMenu could have a NULL callback passed into it. This won't cause any issues until the menu is invoked, in which case it will segfault. Should there be a check when the menu is created, or should the callback be ignored"

[dcnieho]

For this and #44: without looking at the code, having a menu or a timer without a working callback appears useless. We probably shouldn't allow it. Or can anybody see any for of a usage case for this? I will have to look at what GLUT would do in these cases.

[rcmaniac25]

@dcnieho I was thinking of fixing these couple small issues. Do you know what GLUT does?

[dcnieho]

No I don't, could you have a look? Thanks for keeping going! As is probably clear, I do not have time anymore (but will try to keep this fork up to date)