Correctly merge non-empty x-args that do not include queue type

Besides fixing a regression detected by priority_queue_SUITE,
this introduces a drive-by change:

rabbit_priority_queue: avoid an exception when
max priority is a negative value that circumvented validation

Author: michaelklishin

deps/rabbit/src/rabbit_amqqueue.erl

@@ -805,7 +805,7 @@ assert_args_equivalence(Q, NewArgs) ->
 maybe_inject_default_queue_type_shortcut_into_args(Args0, DefaultQueueType) ->
     case rabbit_misc:table_lookup(Args0, <<"x-queue-type">>) of
         undefined ->
-            inject_default_queue_type_shortcut_into_args([], DefaultQueueType);
+            inject_default_queue_type_shortcut_into_args(Args0, DefaultQueueType);
         {longstr, undefined} ->
             %% Important: use a shortcut such as 'quorum' or 'stream' that for the given queue type module
             inject_default_queue_type_shortcut_into_args(Args0, DefaultQueueType);

deps/rabbit/src/rabbit_priority_queue.erl

@@ -129,7 +129,9 @@ priorities(Q) when ?is_amqqueue(Q) ->
             case lists:member(Type, Ints) of
                 false -> none;
                 true  ->
-                    Max = min(RequestedMax, ?MAX_SUPPORTED_PRIORITY),
+                    %% make sure the value is no greater than ?MAX_SUPPORTED_PRIORITY but
+                    %% also is not negative
+                    Max = max(1, min(RequestedMax, ?MAX_SUPPORTED_PRIORITY)),
                     lists:reverse(lists:seq(0, Max))
             end;
         _                    -> none