Sanity check for orphaned ai_holder runtimes

Seems like ai_holder datums take their own sweet time deleting themselves after losing their parent mobs, causing a ton of null holder runtimes during that time window. Now they should be able to sense that their holder mob is gone and avoid that.

code/modules/ai/ai_holder_disabled.dm

@@ -5,6 +5,9 @@
 
 // If our holder is able to do anything.
 /datum/ai_holder/proc/can_act()
+	if(!holder) // Holder missing.
+		SSai.processing -= src
+		return FALSE
 	if(holder.stat) // Dead or unconscious.
 		ai_log("can_act() : Stat was non-zero ([holder.stat]).", AI_LOG_TRACE)
 		return FALSE
@@ -95,4 +98,4 @@
 			holder.IMove(get_step(holder,moving_to))
 			wander_delay = base_wander_delay
 	ai_log("handle_wander_movement() : Exited.", AI_LOG_DEBUG)
-*/
+*/