From argrath: in place_object(object, x, y), validate <x,y> before
using them as indices into level.objects[][] instead of after.
Closes #685
and killed the hero via missile damage rather than its own explosion,
it could trigger an "obj_is_local" panic when end of game cleanup
tried to extinguish it as a light source
+place_object() validated coordinates after using them to index level.objects
Fixes to 3.7.0-x Problems that Were Exposed Via git Repository