Fix GetPreviousOccurrence returning a day in a disallowed month

When the time-of-day search rolls back past the 1st of the month, dayLimit
(and maxDay) can drop below the first valid day of month. The day-of-month
'*' fast path in TryGetPreviousDay returned actualDay = 0, and
DateTimeToTicks(year, month, 0, ...) underflowed into the last day of the
previous month, which is not re-validated against the month field. Guard
against maxDay below the first day of month so the caller advances to the
previous allowed month instead.

src/Cronos/CronExpression.cs

@@ -995,6 +995,20 @@ namespace Cronos
             var lastDayOfMonth = CalendarHelper.GetDaysInMonth(year, month);
             var maxDay = dayLimit > lastDayOfMonth ? lastDayOfMonth : dayLimit;
 
+            // When the time-of-day search rolls back past the 1st of the month, dayLimit
+            // (and therefore maxDay) can drop below the first valid day of month. In that
+            // case there is no matching day left in this month, so the caller must move to
+            // the previous month. Without this guard the day-of-month "*" fast path below
+            // would return actualDay = 0, and DateTimeToTicks(year, month, 0, ...) would
+            // silently underflow into the last day of the previous month, producing an
+            // occurrence in a month that is not allowed by the month field.
+            if (maxDay < CronField.DaysOfMonth.First)
+            {
+                day = default;
+                actualDay = default;
+                return false;
+            }
+
             if (HasFlag(CronExpressionFlag.NearestWeekday))
             {
                 day = HasFlag(CronExpressionFlag.DayOfMonthLast)