checkpatch: fix uninitialized var when run with --no-tree

If checkpatch.pl gets copied out of the tree, --no-tree shouldn't start complaining:

Use of uninitialized value $root in concatenation (.) or string at /path/to/checkpatch.pl line 764.

Let's just give the safe answer instead -- don't warn about "obsolete" files.

---
v2: change condition to check for $root, not $tree

 scripts/checkpatch.pl | 2 ++
 1 file changed, 2 insertions(+)

index a8368d1c4348..c9cbfed326a7 100755
@@ -761,6 +761,8 @@ sub seed_camelcase_file {
 sub is_maintained_obsolete {
 	my ($filename) = @_;
 
+	return 0 if (!defined $root);
+
 	return 0 if (!(-e "$root/scripts/get_maintainer.pl"));
 
 	my $status = `perl $root/scripts/get_maintainer.pl --status --nom --nol --nogit --nogit-fallback -f $filename 2>&1`;

2.8.0.rc3.226.g39d4020