decoder: Really stop decoding on opStop opcode

There was a break already on a `case opStop:` but break inside switch
only breaks out of this switch, not outer construct - looks like it was
a thinko.

So on opStop let's explicitly break out of instruction processing loop.

If we do not do so on e.g "I5\n.I7\n.N." input first call to .Decode()
returns None (from N.), not 5 and second call to Decode() panics (for
this see next patch).

ogorek.go

@@ -119,6 +119,7 @@ func NewDecoder(r io.Reader) Decoder {
 func (d Decoder) Decode() (interface{}, error) {
 
 	insn := 0
+loop:
 	for {
 		insn++
 		key, err := d.r.ReadByte()
@@ -132,7 +133,7 @@ func (d Decoder) Decode() (interface{}, error) {
 		case opMark:
 			d.mark()
 		case opStop:
-			break
+			break loop
 		case opPop:
 			d.pop()
 		case opPopMark:

ogorek_test.go

@@ -78,6 +78,28 @@ func TestDecode(t *testing.T) {
 	}
 }
 
+// test that .Decode() decodes only until stop opcode, and can continue
+// decoding further on next call
+func TestDecodeMultiple(t *testing.T) {
+	input := "I5\n.I7\n.N."
+	expected := []interface{}{int64(5), int64(7), None{}}
+
+	buf := bytes.NewBufferString(input)
+	dec := NewDecoder(buf)
+
+	for i, objOk := range expected {
+		obj, err := dec.Decode()
+		if err != nil {
+			t.Errorf("step #%v: %v", i, err)
+		}
+
+		if !reflect.DeepEqual(obj, objOk) {
+			t.Errorf("step #%v: %q  ; want %q", i, obj, objOk)
+		}
+	}
+}
+
+
 func TestZeroLengthData(t *testing.T) {
 	data := ""
 	output, err := decodeLong(data)