[System] Fix SafeSocketHandle race introduced in 9ae88709dbd3d3ed77e1a6ed3d3493ec14f991fc.

  at System.ThrowHelper.ThrowArgumentOutOfRangeException () [0x00014] in /Users/marek/git/mono/external/referencesource/mscorlib/system/throwhelper.cs:56
  at System.Collections.Generic.List`1[System.Threading.Thread].get_Item (Int32 index) [0x0000e] in /Users/marek/git/mono/external/referencesource/mscorlib/system/collections/generic/list.cs:199
  at System.Net.Sockets.SafeSocketHandle.ReleaseHandle () [0x00052] in /Users/marek/git/mono/mcs/class/System/System.Net.Sockets/SafeSocketHandle.cs:79
  at System.Runtime.InteropServices.SafeHandle.RunRelease () [0x00091] in /Users/marek/git/mono/mcs/class/corlib/System.Runtime.InteropServices/SafeHandle.cs:203
  at System.Runtime.InteropServices.SafeHandle.Dispose (Boolean disposing) [0x00019] in /Users/marek/git/mono/mcs/class/corlib/System.Runtime.InteropServices/SafeHandle.cs:171
  at System.Runtime.InteropServices.SafeHandle.Dispose () [0x00003] in /Users/marek/git/mono/mcs/class/corlib/System.Runtime.InteropServices/SafeHandle.cs:150
  at System.Net.Sockets.Socket.Dispose (Boolean disposing) [0x0004d] in /Users/marek/git/mono/mcs/class/System/System.Net.Sockets/Socket_2_1.cs:1168
  at System.Net.Sockets.Socket.Dispose () [0x00003] in /Users/marek/git/mono/mcs/class/System/System.Net.Sockets/Socket_2_1.cs:1174
  at System.Net.Sockets.Socket.Close () [0x00009] in /Users/marek/git/mono/mcs/class/System/System.Net.Sockets/Socket_2_1.cs:1185
  at System.Net.Sockets.TcpListener.Stop () [0x0000d] in /Users/marek/git/mono/mcs/class/System/System.Net.Sockets/TcpListener.cs:296
  at System.Runtime.Remoting.Channels.Tcp.TcpServerChannel.StopListening (System.Object data) [0x00022] in /Users/marek/git/mono/mcs/class/System.Runtime.Remoting/System.Runtime.Remoting.Channels.Tcp/TcpServerChannel.cs:257
  at System.Runtime.Remoting.Channels.Tcp.TcpChannel.StopListening (System.Object data) [0x00013] in /Users/marek/git/mono/mcs/class/System.Runtime.Remoting/System.Runtime.Remoting.Channels.Tcp/TcpChannel.cs:105
  at System.Runtime.Remoting.Channels.ChannelServices.UnregisterChannel (IChannel chnl) [0x00057] in /Users/marek/git/mono/mcs/class/corlib/System.Runtime.Remoting.Channels/ChannelServices.cs:463
  at MonoTests.Remoting.CallsDomainServer.Stop () [0x00012] in /Users/marek/git/mono/mcs/class/System.Runtime.Remoting/Test/BaseCalls.cs:445
  at (wrapper remoting-invoke-with-check) MonoTests.Remoting.CallsDomainServer:Stop ()
  at (wrapper xdomain-dispatch) MonoTests.Remoting.CallsDomainServer:Stop (object,byte[]&,byte[]&)

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

@@ -75,8 +75,10 @@ namespace System.Net.Sockets {
 					 * We can safely close the socket and throw SocketException in RegisterForBlockingSyscall
 					 * before the blocking system call.
 					 */
-					if (blocking_threads.Count == 1 && blocking_threads[0] == Thread.CurrentThread)
-						break;
+					lock (blocking_threads) {
+						if (blocking_threads.Count == 1 && blocking_threads[0] == Thread.CurrentThread)
+							break;
+					}
 
 					AbortRegisteredThreads ();
 					// Sleep so other threads can resume