vnet: show interface rx-placement may display garbage characters

Before
------
DBGvpp# sh int rx-mode
sh int rx-mode
Thread 1 (vpp_wk_0^@):
  node vmxnet3-input:
    vmxnet3-0/b/0/0 queue 0 (polling)
Thread 2 (vpp_wk_1^@):
  node vmxnet3-input:
    vmxnet3-0/13/0/0 queue 0 (polling)
DBGvpp#

After
-----
DBGvpp# sh int rx-placement
sh int rx-placement
Thread 1 (vpp_wk_0):
  node vmxnet3-input:
    vmxnet3-0/b/0/0 queue 0 (polling)
Thread 2 (vpp_wk_1):
  node vmxnet3-input:
    vmxnet3-0/13/0/0 queue 0 (polling)
DBGvpp#

Change-Id: I5910d502757054c3942fac9d20c5104e95fc6b56
Signed-off-by: Steven <sluong@cisco.com>