Protection lost in expression eval changeover

CheckVarSlotCompatibility contains the comment

	 * Note: we allow a reference to a dropped attribute.  slot_getattr will
	 * force a NULL result in such cases.

While still true, that second sentence is now quite irrelevant, because we
don't go through slot_getattr anymore.  So it seems like we are missing
some needed protection.  I'm inclined to think that it'd be all right to
just throw an error immediately in CheckVarSlotCompatibility if the
target column is dropped.

			regards, tom lane