Fix Null case in byte_stream constructor.

E.g:
xpath_processor::xpath_processor (
   const TiXmlNode * XNp_source_tree,  ///< Source XML tree
   const char * cp_xpath_expr)         ///< XPath expression
      : xpath_stream (cp_xpath_expr)

While calling xpath_stream if cp_xpath_expr is NULL, then it will call byte_stream constructor which will crash in below line:
u_length = strlen (cp_in) + 1;  // strlen(NULL) crashes the code.
So, Null handling done in byte_Stream.