Only bind on tmpsocket once

Since change I8f566543c8da28eb7ed76f1cb80ff4cb2dcbba96 openstackbmc has
failed to start with the error:

  socket.error: [Errno 22] Invalid argument

Moving the early bind so that it is only done when there is no server
specified has fixed the regression for openstackbmc.

Change-Id: Icb16cdc386fb08b54f28b7dff235eb697f1450ee

pyghmi/ipmi/private/session.py

@@ -302,10 +302,10 @@ class Session(object):
         tmpsocket = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)  # INET6
                                     # can do IPv4 if you are nice to it
         tmpsocket.setsockopt(IPPROTO_IPV6, socket.IPV6_V6ONLY, 0)
-        # Rather than wait until send() to bind, bind now so that we have
-        # a port number allocated no matter what
-        tmpsocket.bind(('', 0))
         if server is None:
+            # Rather than wait until send() to bind, bind now so that we have
+            # a port number allocated no matter what
+            tmpsocket.bind(('', 0))
             cls.socketpool[tmpsocket] = 1
         else:
             tmpsocket.bind(server)