Fix GCC warnings when CHECK_LISP_OBJECT_TYPE

* src/lisp.h (lisp_h_Qni): New macro.
(DEFUN): Use it.
* src/alloc.c (syms_of_alloc): Use it.

* src/bytecode.c (Fbyte_code): Fix Lisp_Object/int mixup.

src/bytecode.c

@@ -325,7 +325,7 @@ If the third argument is incorrect, Emacs may crash.  */)
 	 the original unibyte form.  */
       bytestr = Fstring_as_unibyte (bytestr);
     }
-  Lisp_Object fun = CALLN (Fmake_byte_code, 0, bytestr, vector, maxdepth);
+  Lisp_Object fun = CALLN (Fmake_byte_code, Qnil, bytestr, vector, maxdepth);
   return exec_byte_code (fun, 0, 0, NULL);
 }