decode: reject strings with negative length

Found running go-fuzz.

AUTHORS

@@ -4,3 +4,4 @@ Casey Bodley <[email protected]>
 Conrad Pankoff <[email protected]>
 Cenk Alti <[email protected]>
 Jan Winkelmann <[email protected]>
+Patrick Mézard <[email protected]>

decode.go

@@ -238,6 +238,9 @@ func (d *Decoder) decodeString(v reflect.Value) error {
 	if err != nil {
 		return err
 	}
+	if l < 0 {
+		return fmt.Errorf("invalid negative string length: %d", l)
+	}
 
 	//read exactly l bytes out and make our string
 	buf := make([]byte, l)

decode_test.go

@@ -79,6 +79,7 @@ func TestDecode(t *testing.T) {
 		{`di5ei2ee`, new(interface{}), nil, true},
 		{`d3:fooe`, new(interface{}), nil, true},
 		{`l3:foo3:bar`, new(interface{}), nil, true},
+		{`d-1:`, new(interface{}), nil, true},
 	}
 
 	for i, tt := range decodeCases {