Fix broken type comparison assert

Fix broken type comparison assert

In implementing Jason's suggested direction for 93933, the compiler
exploded in a surprising way.  Turns out an assert had been passing
NULLS to comptypes, and therefore not checking what it intended.

Further comptypes, could silently accept such nulls under most
circumstances.

	* class.c (adjust_clone_args): Correct arg-checking assert.
	* typeck.c (comptypes): Assert not nulls.