net/ena/base: fix release of wait event

net/ena/base: fix release of wait event

The wait event is being accessed without making sure it the completion
context exists. The check for that is just below, so it could be used
for releasing wait even safely.

Fixes: 3adcba9a8987 ("net/ena: update HAL to the newer version")
Cc: stable@dpdk.org
Signed-off-by: Michal Krawczyk <mk@semihalf.com>
Reviewed-by: Igor Chauskin <igorch@amazon.com>
Reviewed-by: Guy Tzalik <gtzalik@amazon.com>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>