Fix edge case AttributeError.

Occurs if -r target doesn't exist at all.

BUG=None
TEST=rm -rf $T; cbuildbot <args> -r $T # works.

Change-Id: I8fb958bfb47d8e57c4d02f53a2bb3c67a5c618f4
Reviewed-on: https://gerrit.chromium.org/gerrit/26531
Commit-Ready: Brian Harring <ferringb@chromium.org>
Tested-by: Brian Harring <ferringb@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 6a4eaad..dfceecd 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -1180,9 +1180,10 @@
       stack.Add(osutils.TempDirContextManager, 'cbuildbot-tmp')
       logging.debug("Cbuildbot tempdir is %r.", os.environ.get('TMP'))
 
+    options.preserve_paths = set()
     if log_file is not None:
       stack.Add(tee.Tee, log_file)
-      options.preserve_paths = set([_DEFAULT_LOG_DIR])
+      options.preserve_paths.add(_DEFAULT_LOG_DIR)
 
     if options.cgroups:
       stack.Add(cgroups.SimpleContainChildren, 'cbuildbot')