pkg/go/ast: Avoid doing zero-length writes to the fd.

pkg/go/ast: Avoid doing zero-length writes to the fd.

After each line, ast.Print would do a zero-length write,
which would hit the boundary condition on Plan 9 when
reading over pipes (since message boundaries are
preserved). This change makes sure we only do positive-
length writes.

R=rsc, rminnich, dave, r
CC=golang-dev
https://golang.org/cl/6558046