Prevent a possible divide by 0 in obj/auditor.py.

Fixes bug 941731. If 'elapsed=0' in audit_all_objects set it
to 0.000001 to avoid divide by 0.

Change-Id: If0b43568033167818c482c8927863bea1dccfe57

swift/obj/auditor.py

@@ -105,7 +105,8 @@ class AuditorWorker(object):
                 self.errors = 0
                 self.bytes_processed = 0
             time_auditing += (now - loop_time)
-        elapsed = time.time() - begin
+        # Avoid divide by zero during very short runs
+        elapsed = (time.time() - begin) or 0.000001
         self.logger.info(_(
             'Object audit (%(type)s) "%(mode)s" mode '
             'completed: %(elapsed).02fs. Total quarantined: %(quars)d, '