changed assert with if statement

[accelerated]

In all the testing we have done (and quite extensively) this assert happened only once. However we run debug builds in production and this assert can't happen otherwise the task crashes. I suspect, it's something to do with the underlying rdkafka library where either a message delivery callback was called twice or there was a duplicate ack from kafka, or something like that. I checked the buffered producer code and the pending_acks_ is only incremented in a single place. It could also be that produce() actually sent the message but returned an error afterwards, meaning the pending_acks_ counter was not incremented but kafka still sent an ack back. In any case, this happened only once.

[mfontanini]

This looks like rdkafka's issue, the pending acks tracking here doesn't seem to be wrong. Maybe it's worth raising a bug on their side. Thanks for the fix!

[accelerated]

Ok will do.