preproc: use explicit addressing while clearing smacro

Better point out explicitly that SMacro::next member
is untouched, thus do not use SMacro::next and an array.

CID 1432925

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

asm/preproc.c

@@ -979,7 +979,7 @@ static void clear_smacro(SMacro *s)
 {
     free_smacro_members(s);
     /* Wipe everything except the next pointer */
-    memset(&s->next + 1, 0, sizeof *s - sizeof s->next);
+    memset(&s->name, 0, sizeof(*s) - offsetof(SMacro, name));
 }
 
 /*