Dear all,

the simplification of EXTENDS_TYPE_OF() did not handle the
situation that one of its arguments were a CLASS variable
that was improperly declared.  NULL pointer dereference.

The fix is obvious.  Steve found a similar solution, which
is why I added him as co-author.

Regtested on x86_64-pc-linux-gnu.

Will commit to mainline as obvious within 24 hours unless
there are loud objections.

Thanks,
Harald