Fix NULL dereferences on %substr missing operands

%substr with dangling id issues SIGSEV. Fix it.

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

preproc.c

@@ -3395,12 +3395,13 @@ issue_error:
         tline = expand_smacro(tline->next);
         last->next = NULL;
 
+        if (tline) /* skip expanded id */
             t = tline->next;
         while (tok_type_(t, TOK_WHITESPACE))
             t = t->next;
 
         /* t should now point to the string */
-        if (t->type != TOK_STRING) {
+        if (!tok_type_(t, TOK_STRING)) {
             error(ERR_NONFATAL,
                   "`%%substr` requires string as second parameter");
             free_tlist(tline);