Method make_lex_string

Create a LEX_STRING in this connection. @param lex_str pointer to LEX_STRING object to be initialized @param str initializer to be copied into lex_str @param length length of str, in bytes @param allocate_lex_string if TRUE, allocate new LEX_STRING object, instead of using lex_str value @return NULL on failure, or pointer to the LEX_STRING object