Fix cval assignment exception

For SOL sessions, the queue could be touched without anything in ipmiq.
Have cval be set to something benign in those cases to avoid a trace in
debug.

plugins/hardwaremanagement/ipmi.py

@@ -56,6 +56,7 @@ def _process_chgs(intline):
     global chainpulled
     os.read(intline,1)  # answer the bell
     chainpulled = False
+    cval = ()
     try:
         while ipmiq:
             cval = ipmiq.popleft()