net: remove another unguarded sysfile == nil check
net: remove another unguarded sysfile == nil check

Putting aside the unguarded access to fd.sysfile, the condition will never be true as fd.incref above handles the closed condition.

R=mikioh.mikioh, dvyukov
CC=golang-dev
http://codereview.appspot.com/6845062
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go