Harden function_decl::get_pretty_representation()

Harden function_decl::get_pretty_representation()

This function can abort when called on a function_decl that is not a
member function.  This patch addresses that issue.

	* src/abg-ir.cc (function_decl::get_pretty_representation): Make
	sure the function type is a member function before calling
	get_member_function_is_{virtual,ctor,dtor,const}.

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