Decode: fix index out of range bug (#716)

Fixes #715

decode.go

@@ -400,6 +400,10 @@ func checkAndRemoveUnderscoresIntegers(b []byte) ([]byte, error) {
 		start++
 	}
 
+	if len(b) == start {
+		return b, nil
+	}
+
 	if b[start] == '_' {
 		return nil, newDecodeError(b[start:start+1], "number cannot start with underscore")
 	}

unmarshaler_test.go

@@ -2326,6 +2326,18 @@ func TestIssue710(t *testing.T) {
 	require.Equal(t, map[string]toml.LocalTime{"0": {Nanosecond: 111111111, Precision: 9}}, v2)
 }
 
+func TestIssue715(t *testing.T) {
+	var v interface{}
+	err := toml.Unmarshal([]byte("0=+"), &v)
+	require.Error(t, err)
+
+	err = toml.Unmarshal([]byte("0=-"), &v)
+	require.Error(t, err)
+
+	err = toml.Unmarshal([]byte("0=+A"), &v)
+	require.Error(t, err)
+}
+
 func TestUnmarshalDecodeErrors(t *testing.T) {
 	examples := []struct {
 		desc string