[5.6][boats] fix unsafe entity handling

this probably does not affect normal gameplay, unless you can set boats on fire somehow

mods/ENTITIES/mcl_boats/init.lua

@@ -201,8 +201,9 @@ function boat.on_punch(self, puncher, time_from_last_punch, tool_capabilities, d
 end
 
 function boat.on_step(self, dtime, moveresult)
-	-- ERROR: [#2523] THIS WILL CAUSE INVALID OBJECTREFS
 	mcl_burning.tick(self.object, dtime, self)
+	-- mcl_burning.tick may remove object immediately
+	if not self.object:get_pos() then return end
 
 	self._v = get_v(self.object:get_velocity()) * get_sign(self._v)
 	local v_factor = 1