[Lazarus] Gtk2 division by 0 in themes fix

[Ales Katona]

Btw. one thing which bothers me is that 100% same fpc and c/gcc gtk2 
code resulted in different arguments inside the themeing .so file (I 
compiled my own debug version with printf()). The arguments were from 
some gtkadjustment, but not the one used directly by the trackbar.

For some reason FPC code resulted in slightly different values (except 
one), I have absolutely no idea why since there was a lot of code in 
between the themeing .so and gtk_main() call.

Ales