net/virtio: fix crash on querying xstats
net/virtio: fix crash on querying xstats

Trying to access xstats_names after "if (xstats_names == NULL)" is
obviously wrong, which would result to a crash while running "show
port xstats 0" in testpmd with virtio PMD.

The fix is straightforward; just reverse the check.

Fixes: baf91c395b18 ("net/virtio: fetch extended statistics with integer ids")

Signed-off-by: Yuanhan Liu <5dcef21195e64c9c08aa779bb62b25e9133eaa33@linux.intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk