gtype: fix a no-op assertion
gtype: fix a no-op assertion

g_type_class_add_private() was doing

    g_assert (node->data->instance.private_size <= 0xffff);

but that field is a guint16, so the check was a no-op. (Noticed by
clang, but not gcc for some reason.) Fix it to do the math in a gssize
variable and do the bounds checking there before updating the struct
field.

https://bugzilla.gnome.org/show_bug.cgi?id=706888

mzabaluev/glib,endlessm/glib,mzabaluev/glib,mzabaluev/glib,cosimoc/glib,mzabaluev/glib,cosimoc/glib,ieei/glib,MathieuDuponchelle/glib,tchakabam/glib,Distrotech/glib,MathieuDuponchelle/glib,lukasz-skalski/glib,johne53/MB3Glib,johne53/MB3Glib,gale320/glib,tamaskenez/glib,cention-sany/glib,tamaskenez/glib,endlessm/glib,krichter722/glib,lukasz-skalski/glib,ieei/glib,tchakabam/glib,tamaskenez/glib,tamaskenez/glib,ieei/glib,gale320/glib,lukasz-skalski/glib,lukasz-skalski/glib,cosimoc/glib,johne53/MB3Glib,cention-sany/glib,mzabaluev/glib,lukasz-skalski/glib,Distrotech/glib,cention-sany/glib,gale320/glib,gale320/glib,krichter722/glib,tamaskenez/glib,Distrotech/glib,krichter722/glib,Distrotech/glib,ieei/glib,johne53/MB3Glib,ieei/glib,tchakabam/glib,MathieuDuponchelle/glib,krichter722/glib,endlessm/glib,endlessm/glib,MathieuDuponchelle/glib,endlessm/glib,cention-sany/glib,johne53/MB3Glib,gale320/glib,johne53/MB3Glib,MathieuDuponchelle/glib,tchakabam/glib,Distrotech/glib,krichter722/glib,cosimoc/glib,cosimoc/glib,tchakabam/glib,cention-sany/glib