Fix `burstTracksTargets` repeating weapon aim when target destroyed during burst

[exinfinitum]

Presently, when a gun with burstTracksTargets is aiming at a target that is destroyed during the burst, the gun continues the burst at that given firing angle regardless of the ship's orientation (even if the firing angle is outside the gun's fire arc). This fix makes the gun fire down the default fire angle if the target is destroyed during the burst.

[gmoromisato]

Thank you!