ps/sortformat.c: Double-check chars in verify_short_sort().

To avoid an out-of-bounds access at checkoff[tmp]. The strspn() at the
beginning of the function protects against it already, but double-check
this in case of some future change.

ps/sortformat.c

@@ -428,6 +428,7 @@ static const char *verify_short_sort(const char *arg){
   walk = arg;
   for(;;){
     tmp = *walk;
+    if(tmp < 0 || (size_t)tmp >= sizeof(checkoff)) return _("bad sorting code");
     switch(tmp){
     case '\0':
       return NULL;   /* looks good */