Avoid closing invalid socket after unsuccessful Accept()

Calling wxSocket::Accept() may return an invalid socket, especially when
non-blocking, so don't close it unconditionally.

This avoids an assertion failure in MSVC CRT due to calling
closesocket() with an invalid argument.

src/common/socket.cpp

@@ -529,8 +529,6 @@ wxSocketImpl *wxSocketImpl::Accept(wxSocketBase& wxsocket)
     WX_SOCKLEN_T fromlen = sizeof(from);
     const wxSOCKET_T fd = accept(m_fd, &from.addr, &fromlen);
 
-    wxScopeGuard closeSocket = wxMakeGuard(wxCloseSocket, fd);
-
     // accepting is similar to reading in the sense that it resets "ready for
     // read" flag on the socket
     ReenableEvents(wxSOCKET_INPUT_FLAG);
@@ -538,6 +536,8 @@ wxSocketImpl *wxSocketImpl::Accept(wxSocketBase& wxsocket)
     if ( fd == INVALID_SOCKET )
         return NULL;
 
+    wxScopeGuard closeSocket = wxMakeGuard(wxCloseSocket, fd);
+
     wxSocketManager * const manager = wxSocketManager::Get();
     if ( !manager )
         return NULL;