unix: avoid index out of range in Vmsplice with empty iovs
unix: avoid index out of range in Vmsplice with empty iovs

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 <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

multiplay/golang-sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,golang/sys