checkpatch: don't try to get maintained status when --no-tree is given

Fixes the following warning:

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

Link: http://lkml.kernel.org/r/1476719709-16668-1-git-send-email-jerome.forissier@linaro.org
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -761,7 +761,7 @@ sub seed_camelcase_file {
 sub is_maintained_obsolete {
 	my ($filename) = @_;
 
-	return 0 if (!(-e "$root/scripts/get_maintainer.pl"));
+	return 0 if (!$tree || !(-e "$root/scripts/get_maintainer.pl"));
 
 	my $status = `perl $root/scripts/get_maintainer.pl --status --nom --nol --nogit --nogit-fallback -f $filename 2>&1`;