vpp: fix stdin vs non-interactive command clash

vpp: fix stdin vs non-interactive command clash

In case of both stdin and non-interactive inputs are there
vppctl parses them all, causing mixed corrupted output:
  $ echo foo | vppctl sh bar
  show: unknown input `bar'
  unknown input `foo'

This is not desired, stdin should be ignored if there's a command
but still allow stdin commands - following cases are still equal:
  $ vppctl foo
  $ echo foo | vppctl

Type: fix
Signed-off-by: Vladislav Grishenko <themiron@yandex-team.ru>
Change-Id: I98667391627150c98a57d49ae544e48ef3351f34