SwRTFWriter: fix string conversion

Regression from 02bfed8a (convert
sw/source/filter/ww8/*.cxx from String to OUString, 2013-10-14), check
for string length before invoking OUString::operator[].

This can be reproduced if copy&pasting some text inside a Writer doc
from normal text to rectangle shapes inside group shapes.

Change-Id: Ia8f774b27ca63773792e6af913dfe7e493bea4b8

sw/source/filter/ww8/rtfexport.cxx

@@ -1178,7 +1178,7 @@ SwRTFWriter::SwRTFWriter( const OUString& rFltName, const OUString & rBaseURL )
     SAL_INFO("sw.rtf", OSL_THIS_FUNC);
     SetBaseURL( rBaseURL );
     // export outline nodes, only (send outline to clipboard/presentation)
-    m_bOutOutlineOnly = 'O' == rFltName[0];
+    m_bOutOutlineOnly = (!rFltName.isEmpty() && 'O' == rFltName[0]);
 }
 
 SwRTFWriter::~SwRTFWriter()