CQ: Fix a crash when CMQ master changes in mixed environment test

When policies are applied to change a CMQ master, and that master
moves from 3.13 to 3.12, this triggers a CQ version change
(because it defaults to v2 in 3.13 but v1 in 3.12). which
leads to a crash because the files on disk have an index entry
for messages that are in memory only.

The index entry is only created with CMQs, normal CQs prohibit
this behavior. Since we are going to remove CMQs "soon" the
solution to fix the crash was to simply ignore the index entry
when we know the message is already in memory.

(cherry picked from commit 90e871d)

deps/rabbit/src/rabbit_variable_queue.erl

@@ -1177,7 +1177,11 @@ convert_from_v2_to_v1_loop(QueueName, V1Index0, V2Index0, V2Store0,
                     V1Index1b = rabbit_queue_index:publish(MsgId, SeqId, rabbit_msg_store, Props, IsPersistent, infinity, V1Index1a),
                     rabbit_queue_index:deliver([SeqId], V1Index1b)
             end,
-            {V1Index2, V2Store1}
+            {V1Index2, V2Store1};
+        %% Ignore messages that are in memory and had an entry written in the index.
+        %% @todo Remove this clause some time after CMQs get removed as this will become dead code.
+        ({undefined, _, memory, _, _}, {V1Index1, V2Store1}) ->
+            {V1Index1, V2Store1}
     end, {V1Index0, V2Store0}, Messages),
     %% Flush to disk to avoid keeping too much in memory between segments.
     V1Index = rabbit_queue_index:flush(V1Index3),