During driver reload stress test, after 50-300 reloads occurs panic.
Adding sleeps after load/unload driver issue does not occur.
Too fast load/unload may cause that gt_taskqueue pointer is freed earlier.
Checking null pointer for this fixes it.
Script used to test it:
```
i=1
while true
do
echo "Attempt number $i"
echo "Loading if_ice"
kldload if_ice
echo "Unloading if_ice"
kldunload if_ice
i=$(expr $i + 1)
echo "Last good try: $i" > ice_load_unload_lastGoodTry.txt
done
```