fallback to DefaultEncoding when eCurrentEncoding is unknown
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

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core