Passing an empty iovs []Iovec slice to Vmsplice leads to an index out of
range panic. Fix this by passing an nil unsafe.Pointer to the underlying
syscall in case of an empty slice.

Change-Id: If1844c1b2eb0833de598aed7e79b9fcf061f7975
Reviewed-on: https://go-review.googlesource.com/c/153317
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>