encoding/gob: fix memory corruption

Fixes #3175.

R=golang-dev, iant, rsc, r
CC=golang-dev
https://golang.org/cl/5758069

src/pkg/encoding/gob/decode.go

@@ -707,6 +707,9 @@ func (dec *Decoder) decodeInterface(ityp reflect.Type, state *decoderState, p ui
 	if name == "" {
 		// Copy the representation of the nil interface value to the target.
 		// This is horribly unsafe and special.
+		if indir > 0 {
+			p = allocate(ityp, p, 1) // All but the last level has been allocated by dec.Indirect
+		}
 		*(*[2]uintptr)(unsafe.Pointer(p)) = ivalue.InterfaceData()
 		return
 	}

src/pkg/encoding/gob/gobencdec_test.go

@@ -573,3 +573,22 @@ func TestGobEncodeIsZero(t *testing.T) {
 		t.Fatalf("%v != %v", x, y)
 	}
 }
+
+func TestGobEncodePtrError(t *testing.T) {
+	var err error
+	b := new(bytes.Buffer)
+	enc := NewEncoder(b)
+	err = enc.Encode(&err)
+	if err != nil {
+		t.Fatal("encode:", err)
+	}
+	dec := NewDecoder(b)
+	err2 := fmt.Errorf("foo")
+	err = dec.Decode(&err2)
+	if err != nil {
+		t.Fatal("decode:", err)
+	}
+	if err2 != nil {
+		t.Fatalf("expected nil, got %v", err2)
+	}
+}