Pass empty second argument to dup() call in clone

I don't know if this was there for a reason, but it was making the dup() fail with tcpd, and I don't seem this being handled specially in redoxfs or anywhere else.

This seems to have been the cause for one issue I was having with git; it was somewhat cryptic since the actual call that was failing (the dup itself) wasn't listed in the log of the program's system calls, and I think earlier I missed the fact that the failing read was in another process that had forked.