Boats can only die once.

In certain scenarios a boat can be killed multiple ways in a single tick.
Due to improper guards this can cause the death code to run multiple times
creating item drops. To correct this we insert the appropriate death check.

Remapped by Srg2Source from Bukkit/CraftBukkit@eb3ab5a40453f38a9c08665aafde0a5711afbb72