Converts the [Src, Src+SrcSize) string range from SourceEncoding to DestEncoding and writes it to the [Dest, Dest+DestSize) range. The Src range should contain a null terminator if a null terminator is required in the output. If the Dest range is not big enough to hold the converted output, NULL is returned. In this case, nothing should be assumed about the contents of Dest.