os: Plan 9: avoid doing zero-length writes.

Plan 9 I/O preserves message boundaries, while Go
library code is written for UNIX-like operating
systems which do not. Avoid doing zero-length
writes in package os.

R=rsc, rminnich, ality, rminnich, r
CC=golang-dev
https://golang.org/cl/7406046

src/pkg/os/file_plan9.go

@@ -244,13 +244,23 @@ func (f *File) pread(b []byte, off int64) (n int, err error) {
 
 // write writes len(b) bytes to the File.
 // It returns the number of bytes written and an error, if any.
+// Since Plan 9 preserves message boundaries, never allow
+// a zero-byte write.
 func (f *File) write(b []byte) (n int, err error) {
+	if len(b) == 0 {
+		return 0, nil
+	}
 	return syscall.Write(f.fd, b)
 }
 
 // pwrite writes len(b) bytes to the File starting at byte offset off.
 // It returns the number of bytes written and an error, if any.
+// Since Plan 9 preserves message boundaries, never allow
+// a zero-byte write.
 func (f *File) pwrite(b []byte, off int64) (n int, err error) {
+	if len(b) == 0 {
+		return 0, nil
+	}
 	return syscall.Pwrite(f.fd, b, off)
 }