rabbit_amqqueue: Catch exits when reading classic Q consumers/1

`delegate:invoke/2` catches errors but not exits of the delegate
process. Another process might query for a classic queue's consumers
while the classic queue is being deleted or otherwise terminating and
that would result in an exit of the calling process previously.

(cherry picked from commit a65ceb6)

Author: the-mikedavis

deps/rabbit/src/rabbit_amqqueue.erl

@@ -1519,7 +1519,14 @@ notify_policy_changed(Q) when ?is_amqqueue(Q) ->
 
 consumers(Q) when ?amqqueue_is_classic(Q) ->
     QPid = amqqueue:get_pid(Q),
-    delegate:invoke(QPid, {gen_server2, call, [consumers, infinity]});
+    try
+        delegate:invoke(QPid, {gen_server2, call, [consumers, infinity]})
+    catch
+        exit:_ ->
+            %% The queue process exited during the call.
+            %% Note that `delegate:invoke/2' catches errors but not exits.
+            []
+    end;
 consumers(Q) when ?amqqueue_is_quorum(Q) ->
     QPid = amqqueue:get_pid(Q),
     case ra:local_query(QPid, fun rabbit_fifo:query_consumers/1) of