gtkentry: Call get_*_size() vfuncs unconditionally

gtkentry: Call get_*_size() vfuncs unconditionally

These vfuncs cannot be NULL: implementations are provided by GtkEntry,
and subclasses should not set them to NULL. Instead of conditionalising
the calls to the vfuncs, assert that they’re set and call them
unconditionally.

This prevents the possibility of a subclass setting the vfunc to NULL
and then a gtk_entry_get_*_size() call returning undefined values in its
out variables.

https://bugzilla.gnome.org/show_bug.cgi?id=712760