Fix crash when closing a wxColourDialog under MSW

Check that ms_pfnSetThreadDpiAwarenessContext function pointer was
initialized before using it.

This fixes a regression introduced in 9befda5c26 (Only use
AutoSystemDpiAware when displays have different DPI, 2023-12-10).

See #24121, #24196.

Closes #24288.

include/wx/msw/private/dpiaware.h

@@ -61,7 +61,8 @@ public:
 
     ~AutoSystemDpiAware()
     {
-        if ( ms_pfnSetThreadDpiAwarenessContext )
+        if ( ms_pfnSetThreadDpiAwarenessContext &&
+                ms_pfnSetThreadDpiAwarenessContext != (SetThreadDpiAwarenessContext_t)-1 )
         {
             ms_pfnSetThreadDpiAwarenessContext(m_prevContext);
         }