Single producer / single consumer mempool handle is stored in static
variable and the mempool allocated if stored value is NULL.
If the mempool is freed, NULL should be restored to make sure that
the mempool is allocated once again next time when the test is run.
Fixes: 8ef772aee072 ("app/test: rework mempool test")
Cc: stable@dpdk.org
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
2d645858a9