Here we ICE because early_check_defaulted_comparison passed a null
ctx to same_type_p.  The attached test is ill-formed according to
[class.compare.default]/1, so fixed by detecting this case early.

	PR c++/94478 - ICE with defaulted comparison operator
	* method.c (early_check_defaulted_comparison): Give an error when the
	context is null.

	* g++.dg/cpp2a/spaceship-err4.C: New test.