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, 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.
Merge pull request #353 from J-Shep/351_fix

Fixed hell deaths error #351