Handle nullptr in spawn-monsters-submap

[physics-enthusiast]

Summary

Bugfixes "Handle nullptr in spawn-monsters-submap"

Purpose of change

Fix #70650.
When checking for missions attached to spawning monsters, the case where mission::find fails and returns a nullptr is not handled, presumably leading to a segmentation fault somewhere down the line.

Describe the solution

Add a check for null pointers before proceeding to operate on the returned mission. The debug message for failing to find the mission with the appropriate mission id has been intentionally left in since whatever is causing the failure would still be considered a (separate) bug, it just shouldn't cause a SIGSEGV now.

Describe alternatives you've considered

Testing

Game compiles, no longer segfaults.

Additional context

[physics-enthusiast]

Still preliminary hypothesis, but seems the root cause of #70650's particular missing mission is that if a mission is supposed to spawn monsters but fails before they have spawned (in this case because the mission location never enters the reality bubble before it is declared a failure by an EOC), the submap spawns list (which contains a reference to the mission id) is never updated by set_finish_mission or remove_active_mission and still attempts to spawn monsters by attempting to access the details of a now nonexistent mission.
Quick and dirty fix: pass ok_missing=true to mission::find to silence the error for this particular function call
Proper fix: go through the tiles in the OMT of the mission and remove references to its mission id