fallback to DefaultEncoding when eCurrentEncoding is unknown

as seen when saving abi10092-1 to rtf

sal/rtl/string.cxx:186: OSL_ASSERT: pTarget != NULL && (pSource != NULL ||
nLength == 0) && nLength >= 0 && (nLength == 0 || rtl_isOctetTextEncoding(nEncoding))

Change-Id: I638b94041899145f42ecd80982d8e7c0330b35c0

sw/source/filter/ww8/rtfattributeoutput.cxx

@@ -2074,6 +2074,8 @@ void RtfAttributeOutput::CharFont(const SvxFontItem& rFont)
     m_aStylesEnd.append(OOO_STRING_SVTOOLS_RTF_F);
     m_aStylesEnd.append((sal_Int32)m_rExport.maFontHelper.GetId(rFont));
     m_rExport.eCurrentEncoding = rtl_getTextEncodingFromWindowsCharset(rtl_getBestWindowsCharsetFromTextEncoding(rFont.GetCharSet()));
+    if (m_rExport.eCurrentEncoding == RTL_TEXTENCODING_DONTKNOW)
+        m_rExport.eCurrentEncoding = m_rExport.eDefaultEncoding;
 }
 
 void RtfAttributeOutput::CharFontSize(const SvxFontHeightItem& rFontSize)