Prevent many Mobj related segfaults

Pretty huge mr, but all of those were tried and tested in the Saturn custom client

• 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:)