Examples: DX9: Fix WM_SIZE handling bug introduced in 031e152d2. (#6374)

examples/example_win32_directx9/main.cpp

@@ -99,6 +99,7 @@ int main(int, char**)
         {
             g_d3dpp.BackBufferWidth = g_ResizeWidth;
             g_d3dpp.BackBufferHeight = g_ResizeHeight;
+            g_ResizeWidth = g_ResizeHeight = 0;
             ResetDevice();
         }