Crash when fences get rapidly ignited and extinguished.

[00:03:18]: [string "scripts/components/map.lua"]:76: calling 'GetTileAtPoint' on bad self (number expected, got nil)
LUA ERROR stack traceback:
    =[C]:-1 in (method) GetTileAtPoint (C) <-1--1>
    scripts/components/map.lua:76 in (method) IsOceanTileAtPoint (Lua) <75-78>
    scripts/components/map.lua:81 in (method) IsOceanAtPoint (Lua) <80-84>
    scripts/standardcomponents.lua:31 in (field) onburnt (Lua) <15-41>
    scripts/components/burnable.lua:290 in (field) fn (Lua) <283-307>
    scripts/scheduler.lua:186 in (method) OnTick (Lua) <164-216>
    scripts/scheduler.lua:409 in (global) RunScheduler (Lua) <407-415>
    scripts/update.lua:240 in () ? (Lua) <224-298>

In an attempt to run the code that determines whether a burnt entity should spawn ash or not, the game can input nil coordinates into the IsOceanAtPoint function, leading to a crash.

It seems as simple as checking whether the coordinates exist, and just refusing to spawn ash if not.