fixed xerces xml base64 decode, by typecasting length correctly

Fixed a bug that caused compile error in 32 bit virtual machine. The error was due to not typecasting unsigned long pointer to (XMLSize_t*) , which is a type of size_t , both are 32 bit integers. 
Bug existed in two lines of the same type calling Base64::decode
mis-spell as often as you like.  Almost no one knows how to pronounce it anyway, and I think you know how that feels.