basic/calendarspec: add check for repeat values that would overflow

https://oss-fuzz.com/v2/issue/4651449704251392/7004

src/basic/calendarspec.c

@@ -187,6 +187,8 @@ int calendar_spec_normalize(CalendarSpec *c) {
 }
 
 _pure_ static bool chain_valid(CalendarComponent *c, int from, int to, bool end_of_month) {
+        assert(to >= from);
+
         if (!c)
                 return true;
 
@@ -197,6 +199,10 @@ _pure_ static bool chain_valid(CalendarComponent *c, int from, int to, bool end_
         if (c->start < from || c->start > to)
                 return false;
 
+        /* Avoid overly large values that could cause overflow */
+        if (c->repeat > to - from)
+                return false;
+
         /*
          * c->repeat must be short enough so at least one repetition may
          * occur before the end of the interval.  For dates scheduled