fix: Fix issue of invalid date StdTime dromara#294

Author: kuafuRace

getter.go

@@ -7,6 +7,9 @@ import (
 // StdTime gets standard time.Time.
 // 获取标准 time.Time
 func (c *Carbon) StdTime() time.Time {
+	if c.IsInvalid() {
+		return time.Time{}
+	}
 	if c.loc == nil {
 		return c.time
 	}

getter_unit_test.go

@@ -28,6 +28,13 @@ func TestCarbon_StdTime(t *testing.T) {
 		c := Parse("2020-08-05", PRC)
 		assert.Equal(t, "2020-08-05 00:00:00 +0800 CST", c.StdTime().String())
 	})
+
+	// https://github.com/dromara/carbon/issues/294
+	t.Run("issue294", func(t *testing.T) {
+		assert.Equal(t, "0001-01-01 00:00:00 +0000 UTC", Parse("").StdTime().String())
+		assert.Equal(t, "0001-01-01 00:00:00 +0000 UTC", Parse("0").StdTime().String())
+		assert.Equal(t, "0001-01-01 00:00:00 +0000 UTC", Parse("xxx").StdTime().String())
+	})
 }
 
 func TestCarbon_DaysInYear(t *testing.T) {