app/procinfo: free xstats memory upon failure

Some of the failures cases inside the nic_xstats_display()
function doesn't free the allocated memory for the xstats and
their names, memory is freed now.

Fixes: e2aae1c1 ("ethdev: remove name from extended statistic fetch")
Fixes: 22561383 ("app: replace dump_cfg by proc_info")

Signed-off-by: Reshma Pattan <reshma.pattan@intel.com>
Acked-by: Remy Horton <remy.horton@intel.com>