Fix crash when unregistering unauthenticated connection

lib/rex/server/message_broker.rb

@@ -58,7 +58,7 @@ module Rex
         @connection_message_counter.delete(connection_id)
         @user_connection_associations[
           @connection_user_associations[connection_id]
-        ].delete(connection_id)
+        ]&.delete(connection_id)
         @connection_user_associations.delete(connection_id)
       end
 

spec/server/message_broker_spec.rb

@@ -101,4 +101,14 @@ RSpec.describe Rex::Server::MessageBroker do
       it { is_expected.to be_nil }
     end
   end
+
+  describe "#unregister" do
+    context 'when connection was never associated with a user' do
+      it 'does not crash' do
+        expect do
+          instance.unregister(connection_id_a)
+        end.not_to raise_error
+      end
+    end
+  end
 end