Cleanup functions to detect infinite comparison of class_decl
    
    While looking around, I found some things in need of a cleanup.  This
    patch addresses those.
    
    	* src/abg-ir.cc (class_decl::priv::unmark_as_being_compared): In
    	the overload that takes a pointer to class_decl, re-use the
    	overload that takes a reference.
    	(class_decl::priv::comparison_started): Do not crash if the klass
    	pointer is nil.
    	(equals): In the overload for class_decl&, undef the RESULT macro
    	when it's not used anymore.
    
    Signed-off-by: Dodji Seketeli <dodji@redhat.com>