fix crash on creeper explosion ( #1755 )

mods/ENTITIES/mcl_mobs/api/mob_functions/movement.lua

@@ -38,8 +38,11 @@ end
 --this is a generic float function
 mobs.float = function(self)
 
-	if self.object:get_acceleration().y ~= 0 then
+	local acceleration = self.object:get_acceleration()
+	if acceleration and acceleration.y ~= 0 then
 		self.object:set_acceleration(vector_new(0,0,0))
+	else
+		return
 	end
 
 	local current_velocity = self.object:get_velocity()