Don't compare reference with null in wxBitmap::UngetRawData()

This doesn't seem to make any sense as this function is always called
with non-null "*this" reference from wxPixelData dtor and results in
-Waddress warning from gcc 6.

src/msw/bitmap.cpp

@@ -1374,20 +1374,12 @@ void *wxBitmap::GetRawData(wxPixelDataBase& data, int bpp)
 #endif
 }
 
-void wxBitmap::UngetRawData(wxPixelDataBase& dataBase)
+void wxBitmap::UngetRawData(wxPixelDataBase& WXUNUSED(data))
 {
 #if wxUSE_WXDIB
     if ( !IsOk() )
         return;
 
-    if ( !&dataBase )
-    {
-        // invalid data, don't crash -- but don't assert neither as we're
-        // called automatically from wxPixelDataBase dtor and so there is no
-        // way to prevent this from happening
-        return;
-    }
-
     // if we're a DDB we need to convert DIB back to DDB now to make the
     // changes made via raw bitmap access effective
     if ( !GetBitmapData()->m_isDIB )