Handle special case when disposing a socket.

	When disposing a socket that has more than one asynchronous read/write
	pending, one of the callbacks could call DisposeAllOnClose() while
	another one is waiting on the lock. Once the lock is acquired,
	queue.Count will be 0 and Dequeue would fail.
	This if fixed now.

	Fixes bug #645675.

mcs/class/System/System.Net.Sockets/Socket_2_1.cs

@@ -236,7 +236,11 @@ public void Complete ()
 					SocketAsyncCall sac = null;
 					SocketAsyncResult req = null;
 					lock (queue) {
-						queue.Dequeue (); // remove ourselves
+						// queue.Count will only be 0 if the socket is closed while receive/send
+						// operation(s) are pending and at least one call to this method is
+						// waiting on the lock while another one calls CompleteAllOnDispose()
+						if (queue.Count > 0)
+							queue.Dequeue (); // remove ourselves
 						if (queue.Count > 0) {
 							worker = (Worker) queue.Peek ();
 							if (!Sock.disposed) {