som_alerts_dispatcher: Fix logs when unable to find build.

The previous fix was rushed and logged the wrong thing.

BUG=chromium:804372
TEST=som_alerts_dispatcher

Change-Id: I2557ff3f4c75c188c4bde09fa27e1e837be4255f
Reviewed-on: https://chromium-review.googlesource.com/879221
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: David Riley <davidriley@chromium.org>
Reviewed-by: Jacob Kopczynski <jkop@chromium.org>
diff --git a/scripts/som_alerts_dispatcher.py b/scripts/som_alerts_dispatcher.py
index 359e59e..4dc35dc 100644
--- a/scripts/som_alerts_dispatcher.py
+++ b/scripts/som_alerts_dispatcher.py
@@ -532,19 +532,21 @@
       build_id, severity = build_tuple
       # pylint: enable=unbalanced-tuple-unpacking
       master = db.GetBuildStatus(build_id)
+      if master is None:
+        logging.warn('Could not locate build id %s', build_id)
+        continue
       wfall = master['waterfall']
       build_config = master['build_config']
     elif len(build_tuple) == 3:
       wfall, build_config, severity = build_tuple
       master = db.GetMostRecentBuild(wfall, build_config)
+      if master is None:
+        logging.warn('Could not locate build %s %s', wfall, build_config)
+        continue
     else:
       logging.error('Invalid build tuple: %s' % str(build_tuple))
       continue
 
-    if master is None:
-      logging.warn('Could not locate build %s %s', waterfall, build_config)
-      continue
-
     statuses = [master]
     stages = db.GetBuildStages(master['id'])
     exceptions = db.GetBuildsFailures([master['id']])