Fix uninitialized thread pointer in wxMSW wxJoystick

Initialize m_thread to null to ensure we don't dereference an invalid
pointer later if no joystick with the specified index was found.

Closes https://github.com/wxWidgets/wxWidgets/pull/1663

src/msw/joystick.cpp

@@ -157,6 +157,7 @@ wxJoystick::wxJoystick(int joystick)
     JOYINFO joyInfo;
     int i, maxsticks;
 
+    m_thread = NULL;
     maxsticks = joyGetNumDevs();
     for( i=0; i<maxsticks; i++ )
     {