fix wxStringOutputStream::Write() in Unicode build when the output overlaps a boundary between UTF-8 characters (closes bug 1701426)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45733 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/sstream.h

@@ -61,6 +61,9 @@ public:
     // The stream will write data either to the provided string or to an
     // internal string which can be retrieved using GetString()
     wxStringOutputStream(wxString *pString = NULL)
+#if wxUSE_UNICODE_WCHAR
+        : m_unconv(0)
+#endif // wxUSE_UNICODE_WCHAR
     {
         m_str = pString ? pString : &m_strInternal;
         m_pos = m_str->length() / sizeof(wxChar);
@@ -90,6 +93,11 @@ private:
     wxMBConv m_conv;
 #endif
 
+#if wxUSE_UNICODE_WCHAR
+    // unconverted data from the last call to OnSysWrite()
+    wxMemoryBuffer m_unconv;
+#endif // wxUSE_UNICODE_WCHAR
+
     DECLARE_NO_COPY_CLASS(wxStringOutputStream)
 };