dpkg: fix passing readonly /dev/null fd as stdout/stderr

The read-only /dev/null was duplicated to stdout and stderr, causing writes to those descriptors to fail:
[pid   260] openat(AT_FDCWD, "/dev/null", O_RDONLY) = 7
[pid   260] dup2(7, 0)                  = 0
[pid   260] close(5)                    = 0
[pid   260] dup2(6, 1)                  = 1
[pid   260] dup2(7, 2)                  = 2
[pid   260] write(2, "Chrooting into ", 15) = -1 EBADF (Bad file descriptor)
[pid   260] chroot("/chroot/")          = 0

0 files changed, 0 insertions, 0 deletions