fix ladder physics

When both "forward" and "backward" keys are pressed, now the action
matches the last pressed one. Now the keys work on the ladder exactly
as with regular movement.

source/src/physics.cpp

@@ -504,8 +504,8 @@ void moveplayer(physent *pl, int moveres, bool local, int curtime)
                     if(pl->type==ENT_BOT && pl->state == CS_ALIVE) pl->vel.z = climbspeed; // bots climb upwards only
                     else if(pl->type==ENT_PLAYER && pl->state == CS_ALIVE)
                     {
-                        if(((playerent *)pl)->k_up) pl->vel.z = climbspeed;
-                        else if(((playerent *)pl)->k_down) pl->vel.z = -climbspeed;
+                        if(((playerent *)pl)->k_up && pl->move > 0) pl->vel.z = climbspeed;
+                        else if(((playerent *)pl)->k_down && pl->move < 0) pl->vel.z = -climbspeed;
                     }
                     pl->timeinair = 0;
                 }