os: fix write on Plan 9

In CL 160670043 the write function was changed
so a zero-length write is now allowed. This leads
the ExampleWriter_Init test to fail.

The reason is that Plan 9 preserves message
boundaries, while the os library expects systems
that don't preserve them. We have to ignore
zero-length writes so they will never turn into EOF.

This issue was previously discussed in CL 7406046.

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/163510043