dpif-netdev: Avoid divide by zero.

Otherwise creating the first dpif-netdev bridge fails because there are
no handlers:

  Program terminated with signal 8, Arithmetic exception.
  #0  0x080971e9 in dp_execute_cb (aux_=aux_@entry=0xffcfaa54,
      packet=packet@entry=0xffcfac54, md=md@entry=0xffcfac84,
      a=a@entry=0x8f58930, may_steal=false) at ../lib/dpif-netdev.c:2154
  #1  0x080b5adb in odp_execute_actions__ (dp=dp@entry=0xffcfaa54,
      packet=packet@entry=0xffcfac54, steal=steal@entry=false,
      md=md@entry=0xffcfac84, actions=actions@entry=0x8f58930,
      actions_len=actions_len@entry=20,
      dp_execute_action=dp_execute_action@entry=0x8097040 <dp_execute_cb>,
      more_actions=more_actions@entry=false) at ../lib/odp-execute.c:218
  #2  0x080b5def in odp_execute_actions (dp=dp@entry=0xffcfaa54,
      packet=packet@entry=0xffcfac54, steal=steal@entry=false,
      md=md@entry=0xffcfac84, actions=0x8f58930, actions_len=20,
      dp_execute_action=dp_execute_action@entry=0x8097040 <dp_execute_cb>)
      at ../lib/odp-execute.c:285
  #3  0x08095098 in dp_netdev_execute_actions (actions_len=<optimized out>,
      actions=<optimized out>, md=0xffcfac84, may_steal=false,
      packet=0xffcfac54, key=0xffcfaa5c, dp=<optimized out>)
      at ../lib/dpif-netdev.c:2227
  #4  dpif_netdev_execute (dpif=0x8f59598, execute=0xffcfac78)
      at ../lib/dpif-netdev.c:1551
  #5  0x0809a56c in dpif_execute (dpif=0x8f59598,
      execute=execute@entry=0xffcfac78) at ../lib/dpif.c:1227
  #6  0x08071071 in check_variable_length_userdata (backer=<optimized out>)
      at ../ofproto/ofproto-dpif.c:1040
  #7  open_dpif_backer (backerp=0x8f5834c, type=<optimized out>)
      at ../ofproto/ofproto-dpif.c:921
  #8  construct (ofproto_=0x8f581c0) at ../ofproto/ofproto-dpif.c:1120
  #9  0x080675e0 in ofproto_create (datapath_name=0x8f57310 "br0",
      datapath_type=<optimized out>, ofprotop=ofprotop@entry=0x8f576c8)
      at ../ofproto/ofproto.c:564
  #10 0x080529aa in bridge_reconfigure (ovs_cfg=ovs_cfg@entry=0x8f596d8)
      at ../vswitchd/bridge.c:572
  #11 0x08055e33 in bridge_run () at ../vswitchd/bridge.c:2339
  #12 0x0804cdbd in main (argc=9, argv=0xffcfb554)
      at ../vswitchd/ovs-vswitchd.c:116

This bug was introduced by commit 9bcefb956d8f (ofproto-dpif: fix an ovs
crash when dpif_recv_set returns error).

CC: Andy Zhou <azhou@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>