Provide useful error on bad username

When a bad username was supplied, an attempt to start over
was done without resetting the state machine of the retransmit
facility.  As a consequence, it failed to actually proceed in
restarting the process and would time out rather than returning
the appropriate error.

Change-Id: Ib3207c214fcf96f0dcdb8c4ca3b5387fb58ce1ae

pyghmi/ipmi/private/session.py

@@ -447,6 +447,7 @@ class Session(object):
         self.integrityalgo = 0
         self.k1 = None
         self.rmcptag = 1
+        self.lastpayload = None
         self.ipmicallback = None
         self.sessioncontext = None
         self.sequencenumber = 0