The fallback output can panic when the input shuts down too quickly

[mihaitodor]

Try running this config and press Ctrl+C after the first foo is printed:

input:
  generate:
    count: 0
    mapping: root = ""

output:
  fallback:
    - reject: "mihai"
    - stdout: {}
      processors:
        - sleep:
            duration: 3s
        - log:
            message: foo

shutdown_timeout: 2s

Note that you'll get the following panic:

$ benthos run foo.yaml
INFO Running main config from specified file       @service=redpanda-connect benthos_version="" path=foo.yaml
INFO Listening for HTTP requests at: http://0.0.0.0:4195  @service=redpanda-connect
INFO Input type generate is now active             @service=redpanda-connect label="" path=root.input
INFO Launching a Redpanda Connect instance, use CTRL+C to close  @service=redpanda-connect
INFO Output type stdout is now active              @service=redpanda-connect label="" path=root.output.fallback.1
INFO Output type reject is now active              @service=redpanda-connect label="" path=root.output.fallback.0
INFO foo                                           @service=redpanda-connect custom_source=true label="" path=root.output.fallback.1.processors.1

^CINFO Received SIGINT, the service is closing       @service=redpanda-connect
panic: send on closed channel

goroutine 39 [running]:
github.com/redpanda-data/benthos/v4/internal/impl/pure.(*fallbackBroker).loop.func3({0x109938488, 0x14000d90050}, {0x1098a9038?, 0x140026081b0?})
	/Users/mihaitodor/go/pkg/mod/github.com/redpanda-data/benthos/[email protected]/internal/impl/pure/output_fallback.go:232 +0x134
github.com/redpanda-data/benthos/v4/internal/message.(*Transaction).Ack(0x14002292ac0?, {0x109938488?, 0x14000d90050?}, {0x1098a9038?, 0x140026081b0?})
	/Users/mihaitodor/go/pkg/mod/github.com/redpanda-data/benthos/[email protected]/internal/message/transaction.go:90 +0xd8
github.com/redpanda-data/benthos/v4/internal/component/output.(*AsyncWriter).loop.func4()
	/Users/mihaitodor/go/pkg/mod/github.com/redpanda-data/benthos/[email protected]/internal/component/output/async_writer.go:244 +0xd8
created by github.com/redpanda-data/benthos/v4/internal/component/output.(*AsyncWriter).loop in goroutine 71
	/Users/mihaitodor/go/pkg/mod/github.com/redpanda-data/benthos/[email protected]/internal/component/output/async_writer.go:249 +0x578