Fix bug causing the program to probably go off and free NULL.

src/ll.c

@@ -116,7 +116,7 @@ void* ll_poptail(ll* ll) {
 
 void ll_clear(ll* ll){
   ll_node* head = ll->head;
-  while(ll->head){
+  while(head){
     ll_node* to_free = head;
     head = head->next;
     free(to_free);