null: fix dereference before null check
null: fix dereference before null check

This patch fixes potential null pointer access of eth_null_copy_tx().

Reported-by: John Mcnamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>
Signed-off-by: Tetsuya Mukawa <b347a9f5840345582b30d055e1e99ac6ac7bce3c@igel.co.jp>

venkynv/dpdk-mirror,tsphillips/dpdk-fork,msune/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,msune/dpdk,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,mixja/dpdk,mixja/dpdk,venkynv/dpdk-mirror,mixja/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,mixja/dpdk,venkynv/dpdk-mirror,msune/dpdk,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,msune/dpdk