[TD] Gunboats continue to fixate on target even after it leaves range

[ChthonVII]

The TD gunboats fail to switch targets when their current target goes out of range. The cause is that Target_Something_Nearby() will only alter a valid TarCom if THREAT_RANGE is passed, but THREAT_AREA is used for gunboats.

The fix is to explicitly assign TARGET_NONE as the target before calling Target_Something_Nearby().

Example fix here.