generate disconnection event if recv() returns 0 in Detected_Read() (patch 1730589) [backport from HEAD]

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@46388 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/unix/gsocket.cpp

@@ -1751,6 +1751,12 @@ void GSocket::Detected_Read()
     {
       CALL_CALLBACK(this, GSOCK_CONNECTION);
     }
+    else if (num == 0)
+    {
+      /* graceful shutdown */
+      CALL_CALLBACK(this, GSOCK_LOST);
+      Shutdown();
+    }
     else
     {
       /* Do not throw a lost event in cases where the socket isn't really lost */