net/thunderx: fix build with gcc optimization on

net/thunderx: fix build with gcc optimization on

build error gcc version 6.3.1 20161221 (Red Hat 6.3.1-1),
with EXTRA_CFLAGS="-O3":

.../drivers/net/thunderx/nicvf_ethdev.c:907:9:
   error: ‘txq’ may be used uninitialized in this function
   [-Werror=maybe-uninitialized]
  if (txq->pool_free == nicvf_single_pool_free_xmited_buffers)
      ~~~^~~~~~~~~~~
.../drivers/net/thunderx/nicvf_ethdev.c:886:20:
   note: ‘txq’ was declared here
  struct nicvf_txq *txq;
                    ^~~

Same error on function 'nicvf_eth_dev_init' and 'nicvf_dev_start', it
seems 'nicvf_set_tx_function' inlined when optimization enabled.

Initialize the txq and add NULL check before using it to fix.

Fixes: 7413feee662d ("net/thunderx: add device start/stop and close")
Cc: stable@dpdk.org
Reported-by: Richard Walsh <richard.walsh@intel.com>
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>