lib/shells: Plug econf memory leak

lib/shells: Plug econf memory leak

You can see the memory leak with address sanitizer if util-linux is
compiled with `--with-vendordir=/usr/etc`.

How to reproduce:

1. Prepare a custom shell file as root
```
mkdir -p /etc/shells.d
echo /bin/myshell > /etc/shells.d/custom
```

2. Run chsh as regular user
```
chsh
```

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>