Devserver: fix pylint errors in common_util.py.

BUG=chromium:344939
TEST=Unittests + pylint.

Change-Id: Ic798e58f258019aea563e30e39012db97a4728b4
Reviewed-on: https://chromium-review.googlesource.com/193992
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Chris Sosa <sosa@chromium.org>
Tested-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
diff --git a/autoupdate.py b/autoupdate.py
index ba4ba8f..58c83fa 100644
--- a/autoupdate.py
+++ b/autoupdate.py
@@ -678,9 +678,9 @@
           400, 'hardware_class is required in Omaha Request')
 
     track = app.getAttribute('track')
-    if not track or not track.endswith('-channel'):
+    if not (track and track.endswith('-channel')):
       raise common_util.DevServerHTTPError(
-          400, 'Omaha requests need an update channel')
+          400, 'Omaha requests need a valid update channel')
 
   def _GetStaticUrl(self):
     """Returns the static url base that should prefix all payload responses."""