Passing an empty data []byte to Fsetxattr/Setxattr/Lsetxattr leads
to an index out of range panic. Fix this by passing an nil
unsafe.Pointer to the underlying syscall in this case.

Change-Id: I5ef3983c6304c5317ed5b49afc4ed19791864dbb
Reviewed-on: https://go-review.googlesource.com/c/146198
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>