producer: handle case of no transactions in popTransaction()

[ploxiln]

This corner case generally should not happen, but a nsqd bug or
strange network condition could possibly send erroneous bytes that
are interpreted as a response. The Producer should be robust,
and not panic the whole process.

fixes #345 (hopefully)

[mreiferson]

yea, it makes sense to be defensive here, but let's also log the data so if/when we catch this in the wild we better understand what's happening?

[yiippee]

@ploxiln You said the producer is "thread-safe", but how it is guaranteed to get responses in the order which commands were sent? The nsqd guarantees it? Thanks.

func (w *Producer) popTransaction(frameType int32, data []byte) {
	t := w.transactions[0]  // How can you be sure it's the first one element.  
	w.transactions = w.transactions[1:]
	if frameType == FrameTypeError {
		t.Error = ErrProtocol{string(data)}
	}
	t.finish()
}

[ploxiln]

how it is guaranteed to get responses in the order which commands were sent? The nsqd guarantees it?

Yes, in this way it's similar to http request "pipelining"

[mreiferson]

@ploxiln can you update this as per #346 (comment)

[ploxiln]

OK - how does that look? (first 32 bytes in hex)