Prevent many Mobj related segfaults

-Fix segfaults when damaging mobj with no painstate and when removing source from ShouldDamage
-Fix segfault when removing mobjs while iterating thinglist
-Fix dangling pointer in mapthing after removing mobj
-Fix a segfault when the game cant spawn a MapThing in P_SpawnMapThing
-If P_SetMobjState gets called with a player mobj use P_SetPlayerMobjState instead
-Add a few P_MobjWasRemoved checks
-Make mine explosions a little safer
-Fix rare K_RepairOrbitChain crash (thx indev c:)