When ethtool -X is called without an hkey, ena_com_fill_hash_function()
is called with key=NULL, which is passed to memcpy causing a crash.

This commit fixes this issue by checking key is not NULL.

Fixes: 1738cd3e ("net: ena: Add a driver for Amazon Elastic Network Adapters (ENA)")
Signed-off-by: NSameeh Jubran <sameehj@amazon.com>
Signed-off-by: NArthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>