Multi-turret units may cause assert spam

[KJeff01]

Describe the bug
Using a Dragon body with 2 turrets can cause assert spam. Specifically this line:

warzone2100/src/action.cpp

Line 996 in 806f503

ASSERT_OR_RETURN(, psDroid->psActionTarget[0] != nullptr, "target is NULL while attacking");

It's not exactly new but seeing as these units are somewhat common now I'll make a formal issue for it. If enough units are spamming this assert it could lag the game.

To Reproduce
Steps to reproduce the behavior:

1. Create a Hydra unit (Dragon with 2 turrets).
2. Choose a weapon like the Twin Assault Gun.
3. Choose another weapon like a Vindicator.
4. Place it with the debug menu near something it could sense but not attack up on a hill (most consistent way to trigger it).
5. Let the "sensed" object die. Here it will trigger the assert.
6. If it does happen it may stop if reordering the unit.

Expected behavior
No assert spam.

Your System:

• Game version: master, several releases.

Additional context
Once this is solved we can also enable the 3 turret ultimate scavenger truck again.

[KJeff01]

Closed with #3503.