INT-3457 Fix stop(Runnable) in AbstractEndpoint

JIRA: https://jira.spring.io/browse/INT-3547
JIRA: https://jira.spring.io/browse/INT-3546

INT-3486 changed stop(Runnable) so that it could be
overridden by subclasses, to allow separation of the
`stop()` and `callback` invocation.

However, the refactoring changed the logic such that
the `running` field is not reset, causing `doStop()` to
be called even when the component was not running.

Reset the running field.

Also, the MQTT inbound channel adapter did not test
for a `null` `client` in `doStop()`.