cbc-subdoc upsert command without xattrs sends incorrect path

With upsert subcommand (fulldoc set), it is mandatory to send xattr mutation, to workardound this limitation, cbc-subdoc generates _cbc.version xattr when the user does not specify --xattr switch with upsert. The problem is that scope of the path string is too narrow and it does not live long enough to be sent to network, and the library sending garbage, which results in 0x3d status code.