crash when fake player dig slade

[nonfreegithub]

When a fake player, for example a node, dig slade, it return player ~= nil but it is not a current player, the server_diggable_only function tries to check the privs of the node, then this crashes

We can use the built-in check player:is_player() so the node privs are not checked

This crash occurs, for example, in the technic mod when the corium eats the slade

And can occur in any other mod that uses fake players