Fix expectation of return on bmc reset

Change-Id: I59f0b2dff2792ea704ebd7457f1874e050dfc618

pyghmi/ipmi/command.py

@@ -408,7 +408,7 @@ class Command(object):
     def reset_bmc(self):
         """Do a cold reset in BMC"""
         response = self.raw_command(netfn=6, command=2, retry=False)
-        if 'error' in response:
+        if response and 'error' in response:
             raise exc.IpmiException(response['error'])
 
     def set_bootdev(self,