`nav.um`: Crash when navigating to bottom-right corner

[Vasiliy Tereshkov]

In the rail game, when I click exactly to the bottom-right corner of the screen, I get the following:

error: C:/Users/vtere/Desktop/game/nav.um (79): Index 4080 is out of range 0...4079

        Stack trace:
                C:/Users/vtere/Desktop/game/nav.um:000079: ^Mesh.nav
                C:/Users/vtere/Desktop/game/world.um:000410: ^Player.move
                C:/Users/vtere/Desktop/game/world.um:000423: ^Player.update
                C:/Users/vtere/Desktop/game/world.um:000513: ^World.update
                C:/Users/vtere/Desktop/game/gamemgr.um:000151: ^GameMgr.draw
                C:/Users/vtere/Desktop/game/main.um:000140: <unknown>
                C:/Users/vtere/Desktop/game/signal.um:000058: ^Signal.emit
                C:/Users/vtere/Desktop/game/window.um:000153: umth_frame_callback
                C:/Users/vtere/Desktop/game/tophat_main.um:000007: <unknown>
                ...