Bug 29857 - Don't pop comparison operands that haven't been pushed

Bug 29857 - Don't pop comparison operands that haven't been pushed

While looking at bug
https://sourceware.org/bugzilla/show_bug.cgi?id=29857, I noticed a
crash that is happening due to the fact that the equal overload for
classes tries pop comparison operands that haven't been pushed to the
stack of comparison operands.  Oops.  Fixed thus.

	* src/abg-ir.cc (equals): In the overload for class_or_union,
	don't try to pop, operands that haven't been pushed.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>