Fix mob-in-boat crash in Minetest 5.5-dev

In Minetest 5.4.1, calling get_player_control() on a mob returned the
empty string. Minetest commit 5eb45e1ea03c6104f007efec6dd9c351f310193d
changed this, so now calling get_player_control() on a mob returns nil.

As mcl_boats defines boats that can have a player or a mob as a driver,
code like the following crashes with a changed get_player_control() API:

local ctrl = driver:get_player_control()
if ctrl.sneak then
    detach_object(driver, true)
end

Furthermore, once a world has crashed, joining it near a mob that is the
driver of a boat with such control code immediately crashes again.

When I reported this bug to Minetest, several Minetest core developers
stated that they disliked the old API and proposed other return values
for calling a mob's get_player_control() function – all different from
the empty string. Since I have some doubts that this bug will be fixed
in Minetest 5.5.0, boat code must take into account a nil return value.

Minetest issue: https://github.com/minetest/minetest/issues/11989

mods/ENTITIES/mcl_boats/init.lua

@@ -260,7 +260,7 @@ function boat.on_step(self, dtime, moveresult)
 			return
 		end
 		local yaw = self.object:get_yaw()
-		if ctrl.up then
+		if ctrl and ctrl.up then
 			-- Forwards
 			self._v = self._v + 0.1 * v_factor
 
@@ -269,7 +269,7 @@ function boat.on_step(self, dtime, moveresult)
 				self.object:set_animation({x=0, y=40}, paddling_speed, 0, true)
 				self._animation = 1
 			end
-		elseif ctrl.down then
+		elseif ctrl and ctrl.down then
 			-- Backwards
 			self._v = self._v - 0.1 * v_factor