Fix a file descriptor leak in add_port_hook

Fix a file descriptor leak in add_port_hook

This was probably cut&paste from add_printer_hook which further down has the
unconditional close(fd). In add_port_hook() we're not interested in the output
of 'addport command', so don't create the out fd.
(cherry picked from commit 0c5ca2127ac6e3c71e369242376d27429c3aee5e)
(This used to be commit 9fe09398b79ae7c5e78182112a8cd2c9b5f99ad3)