os: return ErrInvalid if receiver is nil.

Fixes #7043.

Test coming in https://golang.org/cl/46820043

R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/38330045

src/pkg/os/file_posix.go

@@ -144,7 +144,7 @@ func (f *File) Truncate(size int64) error {
 // of recently written data to disk.
 func (f *File) Sync() (err error) {
 	if f == nil {
-		return syscall.EINVAL
+		return ErrInvalid
 	}
 	if e := syscall.Fsync(f.fd); e != nil {
 		return NewSyscallError("fsync", e)