decoder: Don't skip errors in ASCII long format

1. the number has to be terminated with 'L' - check it
   (previously last byte was simply discarded)

2. big.Int.SetString() ok code has to be checked too to catch invalid
   input.

big.Int.SetString() usage in test adjusted to also not skip errors
silently.

ogorek.go

@@ -403,11 +403,15 @@ func (d *Decoder) loadLong() error {
 	if err != nil {
 		return err
 	}
-	if len(line) < 1 {
+	l := len(line)
+	if l < 1 || line[l-1] != 'L' {
 		return io.ErrUnexpectedEOF
 	}
 	v := new(big.Int)
-	v.SetString(string(line[:len(line)-1]), 10)
+	_, ok := v.SetString(string(line[:l-1]), 10)
+	if !ok {
+		return fmt.Errorf("pickle: loadLong: invalid string")
+	}
 	d.push(v)
 	return nil
 }

ogorek_test.go

@@ -13,7 +13,10 @@ import (
 
 func bigInt(s string) *big.Int {
 	i := new(big.Int)
-	i.SetString(s, 10)
+	_, ok := i.SetString(s, 10)
+	if !ok {
+		panic("bigInt")
+	}
 	return i
 }
 
@@ -263,6 +266,10 @@ func TestDecodeError(t *testing.T) {
 		"}g1\n.",
 		"}h\x01.",
 		"}j\x01\x02\x03\x04.",
+
+		// invalid long format
+		"L123\n.",
+		"L12qL\n.",
 	}
 	for _, tt := range testv {
 		buf := bytes.NewBufferString(tt)