Fix player skin changing code.

The issue is that the player model was not unset when he/she leaves the game, and in next join without a server restart, an `if` block in `player_set_model` prevents changing the model and texture again, because the model previously set (which is still in memory in `player_textures` table) and the "new" model are the same. This caused the player's visual to be set to the old 2D sprite.

The issue can be reproduced as follows:
* Start a server and join two clients (one for viewing the other's skin).
* Make one of the clients leave and rejoin. Now the player uses the 2D sprite.

This patch fixes the issue.