Avoid ICE on pmf{} in template.

Avoid ICE on pmf{} in template.

Now that we return the original CONSTRUCTOR from finish_compound_literal,
the call to null_member_pointer_value_p in tsubst_copy_and_build was getting
confused because the CONSTRUCTOR was still empty rather than a valid PMF
value.

	* call.c (null_member_pointer_value_p): Handle an empty CONSTRUCTOR
	of PMF type.

From-SVN: r270324