Win32: correct the 'one-instance' deallocation code

'one-instance' happens to work on Win32 though there are several issues:
- a WM_QUIT is sent to the helper thread when any instance terminates
 (the master or a secondary instance). 'one-instance' should then stop
 working as soon as the first secondary instance terminates.
- But, sending WM_QUIT via SendMessage directly calls the window
procedure callback. And this callback here doesn't process the message
at all. Therefore, it is a no-op and the thread is actually never stopped.

This patch does the following :
- move the WM_QUIT message to ensure that only the master (first) instance
 stops the helper thread.
- process the WM_QUIT message in the window procedure callback, and call
for clean termination of the thread.

Note that PostQuitMessage cannot be directly called as there are two
distincts threads here.

src/win32/specific.c

@@ -310,7 +310,11 @@ static unsigned __stdcall IPCHelperThread( void *data )
 LRESULT CALLBACK WMCOPYWNDPROC( HWND hwnd, UINT uMsg, WPARAM wParam,
                                 LPARAM lParam )
 {
-    if( uMsg == WM_COPYDATA )
+    if( uMsg == WM_QUIT  )
+    {
+        PostQuitMessage( 0 );
+    }
+    else if( uMsg == WM_COPYDATA )
     {
         COPYDATASTRUCT *pwm_data = (COPYDATASTRUCT*)lParam;
         vlc_object_t *p_this;
@@ -382,13 +386,16 @@ void system_End( libvlc_int_t *p_this )
         psz_vlcpath = NULL;
     }
 
+    if (p_helper && p_helper->p_parent == VLC_OBJECT(p_this) )
+    {
+        /* this is the first instance (in a one-instance system)
+         * it is the owner of the helper thread
+         */
         if( ( ipcwindow = FindWindow( 0, L"VLC ipc "VERSION ) ) != 0 )
         {
             SendMessage( ipcwindow, WM_QUIT, 0, 0 );
         }
 
-    if (p_helper && p_helper->p_parent == VLC_OBJECT(p_this) )
-    {
         /* FIXME: thread-safety... */
         vlc_object_detach (p_helper);
         vlc_object_release (p_helper);