Some code assumes that P_SpawnMobj can never return NULL
So spawn MT_RAY in it's place when attempting to spawn MT_NULL and show a console warning
Fixes #800 (closed) and #801 (closed), maybe #802 (closed) also
EXE:srb2win_mobj-spawn-null-fix.exe
Turn the console warning into a devmode print because turns out it happens more often than I thought
Some code assumes that P_SpawnMobj can never return NULL So spawn MT_RAY in it's place when attempting to spawn MT_NULL and show a console warning
