Fixed infinite loop described in INT-3453.

Fixed an infinite loop in the `getNextBuffer()` method of `ChannelInputStream`.

JIRA: https://jira.spring.io/browse/INT-3453

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Yes, Gary, I understand this race condition. I just wanted to say that my sleep is not for avoiding it; it's for the reverse situation you've described. But even without the sleep an assembler cannot exit now, because of the 'while(moreDataAvailable)' loop in `run()`, so it's not a problem anymore.