redir to an empty variable

[Pavel Boldin]

Redirection to an empty variable causes `dash` to exit abnormally via SIGSEGV.

1) $ lsb_release -rd
Description: Ubuntu 16.04.5 LTS
Release: 16.04
2) $ apt-cache policy dash
dash:
  Installed: 0.5.8-2.1ubuntu2
  Candidate: 0.5.8-2.1ubuntu2
  Version table:
 *** 0.5.8-2.1ubuntu2 500
        500 http://ua.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
        100 /var/lib/dpkg/status
3) Script to reproduce:
$ cat test.sh

foobar() {
 local a=$1
 echo abc >&$a
}
foobar

4) Result:
$ dash ~/cloudlinux/kernelcare/test.sh
Segmentation fault (core dumped)