update_suggested_effort: avoid assert if the pqueue has emptied

top_of_rend_pqueue_is_worthwhile requires a nonempty queue.

src/feature/hs/hs_circuit.c

@@ -722,9 +722,9 @@ count_service_rp_circuits_pending(hs_service_t *service)
   return count;
 }
 
-/** Peek at the top entry on the pending rend pqueue. If its level of
- * effort is at least what we're suggesting for that service right now,
- * return 1, else return 0.
+/** Peek at the top entry on the pending rend pqueue, which must not be empty.
+ * If its level of effort is at least what we're suggesting for that service
+ * right now, return 1, else return 0.
  */
 int
 top_of_rend_pqueue_is_worthwhile(hs_pow_service_state_t *pow_state)

src/feature/hs/hs_service.c

@@ -2689,7 +2689,8 @@ update_suggested_effort(hs_service_t *service, time_t now)
     /* If we had a queue during this period, and the current top of queue
      * is at or above the suggested effort, we should re-estimate the effort.
      * Otherwise, it can stay the same (no change to effort). */
-    if (top_of_rend_pqueue_is_worthwhile(pow_state)) {
+    if (smartlist_len(pow_state->rend_request_pqueue) > 0 &&
+        top_of_rend_pqueue_is_worthwhile(pow_state)) {
       pow_state->suggested_effort = (uint32_t)(pow_state->total_effort /
                                                pow_state->rend_handled);
     }