net/txgbe: fix null pointer check

net/txgbe: fix null pointer check

In function cons_parse_ntuple_filter, item->spec and item->mask
should be confirmed not null before use memcmp on it, current
judgement (item->spec || item->mask) just can confirm item->spec
or item->mask is not null, and cause null pointer be used in
memcmp.

This patch fix this problem.

Fixes: b7eeecb17556 ("net/txgbe: parse n-tuple filter")
Cc: stable@dpdk.org
Signed-off-by: Hongbo Zheng <zhenghongbo3@huawei.com>
Signed-off-by: Min Hu (Connor) <humin29@huawei.com>
Acked-by: Jiawen Wu <jiawenwu@trustnetic.com>