Exit poll loop on POLLHUP or anything unexpected

In my opinion, we really shouldn't even get POLLHUP, but
we do. In case we get anything else odd, quite polling on
that fd.

mininet/util.py

@@ -105,7 +105,7 @@ def errRun( *cmd, **kwargs ):
                     err += data
                     if data == '':
                         errDone = True
-            elif event & POLLHUP:
+            else:  # POLLHUP or something unexpected
                 if f == popen.stdout:
                     outDone = True
                 elif f == popen.stderr: