fixed wxString::Last() to run in O(1), not O(n), in UTF-8 build

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45940 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/string.h

@@ -1047,15 +1047,14 @@ public:
     wxUniChar Last() const
     {
       wxASSERT_MSG( !empty(), _T("wxString: index out of bounds") );
-
-          return at(length() - 1);
+      return *rbegin();
     }
 
     // get writable last character
     wxUniCharRef Last()
     {
       wxASSERT_MSG( !empty(), _T("wxString: index out of bounds") );
-          return at(length() - 1);
+      return *rbegin();
     }
 
     /*