encoding/gob: fix panic when decoding []byte to incompatible slice types

Fixes #2662.

R=golang-dev, rogpeppe, r, r
CC=golang-dev, r, rogpeppe
https://golang.org/cl/5515050

src/pkg/encoding/gob/decode.go

@@ -1039,9 +1039,9 @@ func (dec *Decoder) compatibleType(fr reflect.Type, fw typeId, inProgress map[re
 		// Extract and compare element types.
 		var sw *sliceType
 		if tt, ok := builtinIdToType[fw]; ok {
-			sw = tt.(*sliceType)
-		} else {
-			sw = dec.wireType[fw].SliceT
+			sw, _ = tt.(*sliceType)
+		} else if wire != nil {
+			sw = wire.SliceT
 		}
 		elem := userType(t.Elem()).base
 		return sw != nil && dec.compatibleType(elem, sw.Elem, inProgress)

src/pkg/encoding/gob/encoder_test.go

@@ -678,3 +678,11 @@ func TestUnexportedChan(t *testing.T) {
 		t.Fatalf("error encoding unexported channel: %s", err)
 	}
 }
+
+func TestSliceIncompatibility(t *testing.T) {
+	var in = []byte{1, 2, 3}
+	var out []int
+	if err := encAndDec(in, &out); err == nil {
+		t.Error("expected compatibility error")
+	}
+}