metrics: disallow null as metric name
metrics: disallow null as metric name

This patch adds a sanity check so that names passed into
rte_metrics_reg_names() and the wrapper rte_metrics_reg_name()
cannot be NULL.

Fixes: 349950ddb9c5 ("metrics: add information metrics library")
Cc: stable@dpdk.org

Signed-off-by: Remy Horton <34c343e92f0e27f7a8e26a1d8d2146ae7c7dd1a3@intel.com>
Acked-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>

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