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

src/pkg/go/ast/print.go

@@ -108,8 +108,10 @@ func (p *printer) Write(data []byte) (n int, err error) {
 		}
 		p.last = b
 	}
-	m, err = p.output.Write(data[n:])
-	n += m
+	if len(data) > n {
+		m, err = p.output.Write(data[n:])
+		n += m
+	}
 	return
 }