llist.c: Nullify pointers after free()'ing.

* Set all data members to "NULL" in llist_free_inner() after freeing
  them. Altough we normally shouldn't continue working with a list that
  already went through llist_free_inner(), this will protect against
  dangling pointer bugs in case anyone will ever come up with the idea
  of doing so.

* Set list head to "NULL" in llist_free(), basically to put the list
  into an initialized state.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/llist.c

@@ -52,6 +52,8 @@ llist_free (llist_t *l)
       t = i->next;
       mem_free (i);
     }
+
+  l->head = NULL;
 }
 
 void
@@ -62,7 +64,10 @@ llist_free_inner (llist_t *l, llist_fn_free_t fn_free)
   for (i = l->head; i; i = i->next)
     {
       if (i->data)
+        {
           fn_free(i->data);
+          i->data = NULL;
+        }
     }
 }