resultlen Parameter Description for cs_convert Routine

Correction to resultlen parameter description for cs_convert routine in the Open Client and Open Server Common Libraries Reference Manual.

The reference manual incorrectly documents that cs_convert() sets *resultlen to CS_UNUSED when cs_convert() fails; cs_convert() does not set *resultlen to CS_UNUSED upon failure.

Consider the return value of cs_convert() rather than the value of *resultlen to check if the routine has succeeded.