fix(dll): ensure that we check that cur exists every loop iter

ds.c

@@ -132,7 +132,7 @@ void
 		return NULL;
 	}
 
-	for (i = -1, cur = *ll;; i++, cur = cur->next) {
+	for (i = -1, cur = *ll; cur; i++, cur = cur->next) {
 		if (i + 1 == idx) {
 			if (idx == 0) {
 				rm = cur;