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
Oh, plese don't do it that way: you are just asking for bizarro bugs to happen!

The correct fix is to just say   `XMLSize_t length = (XMLSize_t) msg.size(); ` one line higher up. That way, the size of the passed argument actually matches the size of what's being pointed to.