stop accidental leakage of /dev/null fd to child procs

[smemsh]

There was a file descriptor leak introduced by previous commit 1d025fb which opens /dev/null and duplicates it onto the standard input descriptor so child processes do not inherit the tty where X started as their standard input. This PR is a trivial patch to close the leak.

It was noticed like so:

 $ lvm
File descriptor 4 (/dev/null) leaked on lvm invocation. Parent PID 470913: /bin/bash
  WARNING: Running as a non-root user. Functionality may be unavailable.
lvm> ^D

 $ lsa /proc/$$/fd
lrwx------ 1 1001 666 64 20240909004440 0 -> /dev/pts/20
lrwx------ 1 1001 666 64 20240909004440 1 -> /dev/pts/20
lrwx------ 1 1001 666 64 20240909004440 2 -> /dev/pts/20
lrwx------ 1 1001 666 64 20240909004440 255 -> /dev/pts/20
lr-x------ 1 1001 666 64 20240909004440 4 -> /dev/null

the fd 4 is the one from the open("/dev/null") call held by sdorfehs:

 $ lsa /proc/`pgrep -P1 sdorfehs`/fd
lr-x------ 1 1001 666 64 20240909180255 0 -> /dev/null
l-wx------ 1 1001 666 64 20240909180255 1 -> pipe:[11682]
l-wx------ 1 1001 666 64 20240909180255 2 -> pipe:[11682]
lrwx------ 1 1001 666 64 20240909180255 3 -> socket:[12584]
lr-x------ 1 1001 666 64 20240909180255 4 -> /dev/null
lr-x------ 1 1001 666 64 20240909180255 5 -> /home/scott/.config/sdorfehs/bar|
lrwx------ 1 1001 666 64 20240909180255 6 -> socket:[12630]

The patch closes the /dev/null after duplicating, so it won't propagate.