sd-event: remove assert(a<=b) in sleep_between()

With the changes to earliest_time_prioq_compare() to accommodate rate limiting
events, it seems that there can be times where a > b when calculating
sleep_between(). Since the assert() in sleep_between() is no longer true,
remove it and swap a and b when a > b and continue to pick a time between
the two as usual.

src/libsystemd/sd-event/sd-event.c

@@ -2903,7 +2903,10 @@ static int event_source_leave_ratelimit(sd_event_source *s) {
 static usec_t sleep_between(sd_event *e, usec_t a, usec_t b) {
         usec_t c;
         assert(e);
-        assert(a <= b);
+
+        /* swap a and b such that we always have a <= b */
+        if (a > b)
+                SWAP_TWO(a, b);
 
         if (a <= 0)
                 return 0;