time: fix custom_format panic (fix #24977) (#24987)

Author: kbkpbot

vlib/time/format.v

@@ -401,10 +401,20 @@ pub fn (t Time) custom_format(s string) string {
 				sb.write_string(ordinal_suffix(t.month))
 			}
 			'MMM' {
-				sb.write_string(long_months[t.month - 1][0..3])
+				m := if t.month >= 1 && t.month <= 12 {
+					long_months[t.month - 1][0..3]
+				} else {
+					long_months[0][0..3]
+				}
+				sb.write_string(m)
 			}
 			'MMMM' {
-				sb.write_string(long_months[t.month - 1])
+				m := if t.month >= 1 && t.month <= 12 {
+					long_months[t.month - 1]
+				} else {
+					long_months[0]
+				}
+				sb.write_string(m)
 			}
 			'D' {
 				sb.write_string(t.day.str())

vlib/time/time_test.v

@@ -476,3 +476,9 @@ fn test_parse_weekday() {
 	tm_s := tm.custom_format(format)
 	assert tm_s == 'Thursday Jan 01 00:00:00 1970'
 }
+
+fn test_empty_time() {
+	t := time.Time{}
+	assert t.unix() == -62169984000
+	assert t.custom_format('MMMM YYYY') == 'January 0'
+}