re PR c++/20746 ([4.0 only] Incorrect return value for covariant return function returning null ptr)

cp:
	PR c++/20746
	* method.c (use_thunk): Protect covariant pointer return
	adjustments from NULL pointers.
testsuite:
	PR c++/20746
	* g++.dg/abi/covariant5.C: New.

From-SVN: r97557