bytes.Buffer.ReadFrom: fix bug.
Fixes #852.

R=rsc
CC=golang-dev
https://golang.org/cl/1680042
diff --git a/src/pkg/bytes/buffer.go b/src/pkg/bytes/buffer.go
index 32571d8..01e6aef 100644
--- a/src/pkg/bytes/buffer.go
+++ b/src/pkg/bytes/buffer.go
@@ -134,7 +134,7 @@
 			b.off = 0
 		}
 		m, e := r.Read(b.buf[len(b.buf):cap(b.buf)])
-		b.buf = b.buf[b.off : len(b.buf)+m]
+		b.buf = b.buf[0 : len(b.buf)+m]
 		n += int64(m)
 		if e == os.EOF {
 			break