fix: prevent race condition when manually closing channel

In some situations, explicitly calling `Channel#close` might unregister
the channel from the session before the AMQP Close message gets processed.

Backtrace:

```
Dec 15 16:39:02 ruby[321979]: W, [2022-12-15T16:39:02.427534 #321979]  WARN -- #<Bunny::Session:0x48648 app@localhost:5672, vhost=/, addresses=[localhost:5672]>: TCP connection failed, reconnecting in 5.0 seconds
Dec 15 16:39:02 ruby[321979]: W, [2022-12-15T16:39:02.427582 #321979]  WARN -- #<Bunny::Session:0x48648 app@localhost:5672, vhost=/, addresses=[localhost:5672]>: Will recover from a network failure (no retry limit)...
Dec 15 16:39:02 ruby[321979]: #<Thread:0x00007f3c1ae18540 /opt/app/vendor/bundle/ruby/3.0.0/gems/bunny-2.19.0/lib/bunny/reader_loop.rb:35 run> terminated with exception (report_on_exception is true):
Dec 15 16:39:02 ruby[321979]: /opt/app/vendor/bundle/ruby/3.0.0/gems/bunny-2.19.0/lib/bunny/session.rb:1031:in `block in unregister_channel': undefined method `number' for nil:NilClass (NoMethodError)
Dec 15 16:39:02 ruby[321979]:         from /opt/app/vendor/bundle/ruby/3.0.0/gems/bunny-2.19.0/lib/bunny/session.rb:1030:in `synchronize'
Dec 15 16:39:02 ruby[321979]:         from /opt/app/vendor/bundle/ruby/3.0.0/gems/bunny-2.19.0/lib/bunny/session.rb:1030:in `unregister_channel'
Dec 15 16:39:02 ruby[321979]:         from /opt/app/vendor/bundle/ruby/3.0.0/gems/bunny-2.19.0/lib/bunny/session.rb:675:in `ensure in handle_frame'
Dec 15 16:39:02 ruby[321979]:         from /opt/app/vendor/bundle/ruby/3.0.0/gems/bunny-2.19.0/lib/bunny/session.rb:675:in `handle_frame'
Dec 15 16:39:02 ruby[321979]:         from /opt/app/vendor/bundle/ruby/3.0.0/gems/bunny-2.19.0/lib/bunny/reader_loop.rb:92:in `run_once'
Dec 15 16:39:02 ruby[321979]:         from /opt/app/vendor/bundle/ruby/3.0.0/gems/bunny-2.19.0/lib/bunny/reader_loop.rb:39:in `block in run_loop'
Dec 15 16:39:02 ruby[321979]:         from /opt/app/vendor/bundle/ruby/3.0.0/gems/bunny-2.19.0/lib/bunny/reader_loop.rb:36:in `loop'
Dec 15 16:39:02 ruby[321979]:         from /opt/app/vendor/bundle/ruby/3.0.0/gems/bunny-2.19.0/lib/bunny/reader_loop.rb:36:in `run_loop'
```

lib/bunny/session.rb

@@ -671,8 +671,12 @@ def handle_frame(ch_number, method)
           # avoid doing that while holding a mutex lock. MK.
           ch.handle_method(method)
         ensure
-          # synchronises on @channel_mutex under the hood
-          self.unregister_channel(ch)
+          if ch.nil?
+            @logger.warn "Could not associate Close message with channel. Probably it was already closed."
+          else
+            # synchronises on @channel_mutex under the hood
+            self.unregister_channel(ch)
+          end
         end
       when AMQ::Protocol::Basic::GetEmpty then
         ch = find_channel(ch_number)