os.ReadAt doesn't return EOF at EOF.

thanks to lionkov for the fix.

Fixes #262.

R=rsc
CC=golang-dev
https://golang.org/cl/156097

src/pkg/os/file.go

@@ -138,6 +138,9 @@ func (file *File) ReadAt(b []byte, off int64) (n int, err Error) {
 	}
 	for len(b) > 0 {
 		m, e := syscall.Pread(file.fd, b, off);
+		if m == 0 && e == 0 {
+			return n, EOF
+		}
 		n += m;
 		if e != 0 {
 			err = &PathError{"read", file.name, Errno(e)};