fix segfault

So basically, I used i++ on a linked list instead of i->next. As if that
wasn't bad enough on its own, it even managed to somehow still work in the 2
test scenarios I gave it. Regardless, it is now fixed.

parse.c

@@ -395,7 +395,7 @@ static void stmt(State *s, Scope *sc, TokListItem *t) {
 
 		/* find beginning of while loop body */
 		TokListItem *lcurl;
-		for (TokListItem *i = t;; i++) {
+		for (TokListItem *i = t;; i = i->next) {
 			if (i == NULL) {
 				mark_err(&start->tok);
 				set_err("Expected '{' after 'while' loop condition");