check for NULL when attaching to foreground window

Should fix Issue #66
thanks to joerg.bornemann for the patch

lib/platform/CSynergyHook.cpp

@@ -156,6 +156,9 @@ attachThreadToForeground()
 	// already in the right thread.
 	if (g_hookThread != 0) {
 		HWND window    = GetForegroundWindow();
+        if (window == NULL)
+            return false;
+
 		DWORD threadID = GetWindowThreadProcessId(window, NULL);
 		// skip if no change
 		if (g_attachedThread != threadID) {