Bail out of P_RespawnSpecials if no mobjinfo entry has a doomednum that...

Bail out of P_RespawnSpecials if no mobjinfo entry has a doomednum that matches, and print a warning to the console.

This prevents a mobj of NUMMOBJTYPES from being created and potentially crashing the game. Really, this happens only if you start messing with the mapthings of respawning items like rings/weapons/etc in the first place via Lua.