Fix a number of grab bugs and odd behaviour still around.

This commit fixes a number of bugs with mob grabbing code.
Primary things fixed:
 - Aliens grabbing people will no longer break horribly, they call
   grabbedby like everything else now.
 - Any nulls that do manage to find their way into a grab list will be
   cleaned up by the next grab on that person.
 - Fixed a number of odd behaviours when grabbing xenomorphs, see
   bbcc19c764 for more details.
 - Fixed grabs having a tendency to shift someone's pixel_xy and not reset
   it when being deleted. This was simply because it previously relied on
   del() to stop all procs instantly.

code/modules/mob/living/carbon/human/human_attackalien.dm

@@ -7,20 +7,7 @@
 		if ("help")
 			visible_message(text("\blue [M] caresses [src] with its scythe like arm."))
 		if ("grab")
-			if(M == src || anchored)
-				return
-			if (w_uniform)
-				w_uniform.add_fingerprint(M)
-			var/obj/item/weapon/grab/G = new /obj/item/weapon/grab(M, src)
-
-			M.put_in_active_hand(G)
-
-			grabbed_by += G
-			G.synch()
-			LAssailant = M
-
-			playsound(loc, 'sound/weapons/thudswoosh.ogg', 50, 1, -1)
-			visible_message(text("\red [] has grabbed [] passively!", M, src))
+			grabbedby(M)
 
 		if("harm")
 			M.do_attack_animation(src)

code/modules/mob/mob_grab.dm

@@ -27,8 +27,13 @@
 	w_class = 5.0
 
 
-/obj/item/weapon/grab/New(mob/user, mob/victim)
+/obj/item/weapon/grab/New(var/mob/user, var/mob/victim)
 	..()
+
+	//Okay, first off, some fucking sanity checking. No user, or no victim, or they are not mobs, no grab.
+	if(!istype(user) || !istype(victim))
+		return
+
 	loc = user
 	assailant = user
 	affecting = victim
@@ -52,8 +57,25 @@
 				G.dancing = 1
 				G.adjust_position()
 				dancing = 1
+
+	clean_grabbed_by(assailant, affecting)
 	adjust_position()
 
+/obj/item/weapon/grab/proc/clean_grabbed_by(var/mob/user, var/mob/victim) //Cleans up any nulls in the grabbed_by list.
+	if(istype(user))
+
+		for(var/entry in user.grabbed_by)
+			if(isnull(entry) || !istype(entry, /obj/item/weapon/grab)) //the isnull() here is probably redundant- but it might outperform istype, who knows. Better safe than sorry.
+				user.grabbed_by -= entry
+
+	if(istype(victim))
+
+		for(var/entry in victim.grabbed_by)
+			if(isnull(entry) || !istype(entry, /obj/item/weapon/grab))
+				victim.grabbed_by -= entry
+
+	return 1
+
 //Used by throw code to hand over the mob, instead of throwing the grab. The grab is then deleted by the throw code.
 /obj/item/weapon/grab/proc/get_mob_if_throwable()
 	if(affecting && assailant.Adjacent(affecting))
@@ -73,7 +95,7 @@
 
 
 /obj/item/weapon/grab/process()
-	confirm()
+	if(!confirm())	return //If the confirm fails, the grab is about to be deleted. That means it shouldn't continue processing.
 
 	if(assailant.client)
 		if(!hud)	return //this somehow can runtime under the right circumstances