Fixed hell deaths bug #351

In die.lua:die_tick_reaper:tryHellDeathSpawnScenario() I had not included responses to the possibility of this function's findIdleTile() calls returning nil tile coordinates when it couldn't find idle tiles, so in this commit I've simply added two IF statement conditions to check that there are no nil tile coordinates before they are used.

```
if hole_x and humanoid.world:canNonSideObjectBeSpawnedAt(...) then
```
If the hole_x coordinate is nil then the call after this condition's AND statement won't be made.