Fix falling through nodes on world load (fixes #2784)

On world load the collision code can not see node boxes, since the
nodes have not been loaded. Thus it collided only at the next full
node. However, standing on a slab on world load leaded to sinking into
it until the world finished loading. Then one maybe fell further, if
the node below was not walkable.

Now, with this commit, when no node around the player has been loaded
it simply does not move the player.