agent: keep a ref on the agent while removal of TURN refreshes

The patch makes the agent alive at least until the last callback of
the removal of turn refreshes when a stream is deleted from the agent.

Closes #84

agent/agent.c

@@ -3285,6 +3285,8 @@ on_stream_refreshes_pruned (NiceAgent *agent, NiceStream *stream)
 
   nice_stream_close (agent, stream);
 
+  g_object_unref (agent);
+
   agent_unlock (agent);
 
   /* Actually free the stream. This should be done with the lock released, as
@@ -3319,6 +3321,8 @@ nice_agent_remove_stream (
     return;
   }
 
+  g_object_ref (agent);
+
   /* note: remove items with matching stream_ids from both lists */
   conn_check_prune_stream (agent, stream);
   discovery_prune_stream (agent, stream_id);