Fix possible boat crash

mods/rp_boats/init.lua

@@ -105,7 +105,7 @@ local register_boat = function(name, def)
 			if self._driver then
 				if not self._driver:is_player() then
 					self._driver = nil
-				end
+				else
 					local ctrl = self._driver:get_player_control()
 					if ctrl.left and not ctrl.right then
 						yaw = yaw + YAW_CHANGE_RATE * dtime
@@ -122,6 +122,7 @@ local register_boat = function(name, def)
 						end
 					end
 				end
+			end
 			local get_horvel = function(v, yaw)
 				local x = -math.sin(yaw) * v
 				local z = math.cos(yaw) * v