* lisp.h (TAG_SYMPTR): Don't do arithmetic on NULL.

This is a followup to the "Port Qnil==0 XUNTAG to clang" patch.
Although clang doesn't need it, some other compiler might, and
it's easy enough to be safe.

src/ChangeLog

 2015-01-08  Paul Eggert  <eggert@cs.ucla.edu>
+	* lisp.h (TAG_SYMPTR): Don't do arithmetic on NULL.
+	This is a followup to the "Port Qnil==0 XUNTAG to clang" patch.
+	Although clang doesn't need it, some other compiler might, and
+	it's easy enough to be safe.
+
 	* conf_post.h (ATTRIBUTE_ALLOC_SIZE): Port to clang 3.5.0.
 	Apparently clang removed support for the alloc_size attribute.

src/lisp.h

@@ -729,7 +729,8 @@ struct Lisp_Symbol
 
 /* Yield an integer that tags PTR as a symbol.  */
 #define TAG_SYMPTR(ptr) \
-  TAG_PTR (Lisp_Symbol, (char *) (ptr) - (char *) (USE_LSB_TAG ? lispsym : 0))
+  TAG_PTR (Lisp_Symbol, \
+	   USE_LSB_TAG ? (char *) (ptr) - (char *) lispsym : (intptr_t) (ptr))
 
 /* Declare extern constants for Lisp symbols.  These can be helpful
    when using a debugger like GDB, on older platforms where the debug