Calling a non-existent font number causes a segmentation fault

[kojix2]

Hi @jheinen and GR.jl developers!

Thank you for the new font setting feature. I ported the font setting function to GR.rb and found the following error.

import GR
GR.settextfontprec(235,3)
GR.text(0.1, 0.1, "blabla") # Segmentation fault

As you probably already know, but I respectfully report to you.