authzone: fix memory leak in xfer_set_masters() error path (#1480)

Added memory deallocation for the `file` and `host` fields of the
`auth_master` node in the event of a URL/allocation error, and
unlinked the partially created node from the masters list by
resetting the link that pointed to it.

Signed-off-by: Petr Vaganov <petrvaganoff@gmail.com>