gtk: Fix a small issue in the previous commit to IconTheme
gtk: Fix a small issue in the previous commit to IconTheme

The native array has an extra NULL element, so we pass the length of the
managed array, which is the real length of the array.

akrisiun/gtk-sharp,sillsdev/gtk-sharp,openmedicus/gtk-sharp,Gankov/gtk-sharp,antoniusriha/gtk-sharp,orion75/gtk-sharp,openmedicus/gtk-sharp,antoniusriha/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,antoniusriha/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp,openmedicus/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,Gankov/gtk-sharp,antoniusriha/gtk-sharp,openmedicus/gtk-sharp,Gankov/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,sillsdev/gtk-sharp,orion75/gtk-sharp,akrisiun/gtk-sharp,antoniusriha/gtk-sharp,orion75/gtk-sharp,akrisiun/gtk-sharp