preproc.: Fix NULL dereference on broken %strlen argument

Under particular circumstances %strlen may cause SIGSEG. A typical
example is %strlen with nonexistent macro argument.

[ Testcase test/strlen.asm ]

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

preproc.c

@@ -3254,7 +3254,7 @@ issue_error:
         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,
                   "`%%strlen` requires string as second parameter");
             free_tlist(tline);

test/strlen.asm

@@ -0,0 +1,5 @@
+%macro strlen_test 1
+    %strlen len %2 ; not existing argument
+%endmacro
+
+strlen_test 'a'