clisp: avoid using NIL for null pointers.

- Work around FFI:C-POINTER in %MEM-REF, %FOREIGN-FUNCALL and %DEFCALLBACK.
- Don't accept NIL in POINTERP, FOREIGN-POINTER or INC-POINTER.
- New tests: CALLBACKS.NIL-FOR-NULL, FUNCALL.POINTER-NOT-NIL, POINTER.NULL
  and FOREIGN-POINTER-TYPE.NIL.