Add if statement to remote.Close() call

[B3rs]

After an error in listener.Accept() remote object can be nil.

rend/server/listen.go

Line 122 in d3db570

remote, err := listener.Accept()

Calling remote.Close() in this line causes a panic.

rend/server/listen.go

Line 125 in d3db570

remote.Close()

We noticed this behaviour because this error Error accepting connection from remote: accept tcp [::]:11211: accept4: too many open files causes our implementation to panic.

Since the wanted behavior is to continue the listener.Accept() loop in case of error, we should wrap this particular remote.Close() call with an if remote != nil statement

Fixes #129