copy: fix a segfault in SELinux context copying code

* src/selinux.c (restorecon_private): On ArchLinux the
`fakeroot cp -a file1 file2` command segfaulted due
to getfscreatecon() returning a NULL context.
So map this to the sometimes ignored ENODATA error,
rather than crashing.
* tests/cp/no-ctx.sh: Add a new test case.
* tests/local.mk: Reference the new test.
* NEWS: Mention the fix.
Fixes http://bugs.gnu.org/16335

src/selinux.c

@@ -192,6 +192,11 @@ restorecon_private (char const *path, bool local)
     {
       if (getfscreatecon (&tcon) < 0)
         return rc;
+      if (!tcon)
+        {
+          errno = ENODATA;
+          return rc;
+        }
       rc = lsetfilecon (path, tcon);
       freecon (tcon);
       return rc;