ls: fix crash printing SELinux context for unstatable files

ls: fix crash printing SELinux context for unstatable files

This crash was identified by Cyber Independent Testing Lab:
https://cyber-itl.org/2020/10/28/citl-7000-defects.html
and was introduced with commit v6.9.90-11-g4245876e2

* src/ls.c (gobble_file): Ensure scontext is initialized
in the case where files are not statable.
* tests/ls/selinux-segfault.sh: Renamed from proc-selinux-segfault.sh,
and added test case for broken symlinks.
* tests/local.mk: Adjust for the renamed test.
* NEWS: Mention the bug fix.