app/testpmd: check statistics query before printing

app/testpmd: check statistics query before printing

In function 'fwd_stats_display', if function 'rte_eth_stats_get' fails,
'stats' is uncertainty value. The display result will be abnormal.

This patch check the return value of 'rte_eth_stats_get' to avoid
display abnormal stats.

Fixes: 53324971a14e ("app/testpmd: display/clear forwarding stats on demand")
Cc: stable@dpdk.org
Signed-off-by: Min Hu (Connor) <humin29@huawei.com>
Acked-by: Aman Singh <aman.deep.singh@intel.com>