Fix access to ramp pointer ahead of it being acquired.
Fix access to ramp pointer ahead of it being acquired.

As far as I can tell, this was an accident when refactoring a while ago
to get better C89 compatibility -- the end result is that a variable
initialization got moved but I forgot to initialize it in the right
place after moving it. D'oh.

Closes #7.

nilium/ruby-glfw3,nilium/ruby-glfw3