gobs: fix bug in singleton arrays

Fixes #934.

R=rsc
CC=golang-dev
https://golang.org/cl/1869043

src/pkg/gob/encode.go

@@ -722,10 +722,6 @@ func encOpFor(rt reflect.Type) (encOp, int, os.Error) {
 				return nil, 0, err
 			}
 			op = func(i *encInstr, state *encoderState, p unsafe.Pointer) {
-				slice := (*reflect.SliceHeader)(p)
-				if slice.Len == 0 {
-					return
-				}
 				state.update(i)
 				state.err = encodeArray(state.b, uintptr(p), elemOp, t.Elem().Size(), indir, t.Len())
 			}

src/pkg/gob/encoder_test.go

@@ -274,6 +274,7 @@ var testFloat32 float32
 var testString string
 var testSlice []string
 var testMap map[string]int
+var testArray [7]int
 
 type SingleTest struct {
 	in  interface{}
@@ -287,6 +288,8 @@ var singleTests = []SingleTest{
 	SingleTest{"bike shed", &testString, ""},
 	SingleTest{[]string{"bike", "shed", "paint", "color"}, &testSlice, ""},
 	SingleTest{map[string]int{"seven": 7, "twelve": 12}, &testMap, ""},
+	SingleTest{[7]int{4, 55, 0, 0, 0, 0, 0}, &testArray, ""}, // case that once triggered a bug
+	SingleTest{[7]int{4, 55, 1, 44, 22, 66, 1234}, &testArray, ""},
 
 	// Decode errors
 	SingleTest{172, &testFloat32, "wrong type"},
@@ -320,7 +323,7 @@ func TestSingletons(t *testing.T) {
 		// Get rid of the pointer in the rhs
 		val := reflect.NewValue(test.out).(*reflect.PtrValue).Elem().Interface()
 		if !reflect.DeepEqual(test.in, val) {
-			t.Errorf("decoding int: expected %v got %v", test.in, val)
+			t.Errorf("decoding singleton: expected %v got %v", test.in, val)
 		}
 	}
 }