fix comparison of BYTE variable with -1 after last commit

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@51030 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/window.cpp

@@ -5938,7 +5938,7 @@ WXWORD wxCharCodeWXToMSW(int wxk, bool *isVirtual)
         default:
             // check to see if its one of the OEM key codes.
             BYTE vks = LOBYTE(VkKeyScan(wxk));
-            if ( vks != -1 )
+            if ( vks != 0xff )
             {
                 vk = vks;
             }