crashtesting: plain-text import allows liberal with the UTF-8 input

see ImplConvertUtf8ToUnicode which allows surrogates, so we need a non-liberal
utf-input there if we want to enfore non-liberal utf-output here.

Change-Id: I713818d7b6d79922d9b08f2b137a803384eccb65

sax/source/tools/fastserializer.cxx

@@ -94,7 +94,7 @@ namespace sax_fastparser {
 
     void FastSaxSerializer::write( const OUString& sOutput, bool bEscape )
     {
-        write( sOutput.toUtf8(), bEscape );
+        write( OUStringToOString(sOutput, RTL_TEXTENCODING_UTF8), bEscape );
 
     }