Handle cases where the headers table is undefined

Author: michaelklishin

src/rabbit_dead_letter.erl

@@ -112,6 +112,8 @@ group_by_queue_and_reason(Tables) ->
           end, {sets:new(), []}, Tables),
     Grouped.
 
+update_x_death_header(Info, undefined) ->
+    update_x_death_header(Info, []);
 update_x_death_header(Info, Headers) ->
     X = x_death_event_key(Info, <<"exchange">>),
     Q = x_death_event_key(Info, <<"queue">>),