Always reset the buffer on flush

[hcarty]

Otherwise the buffer contents can be overwritten after a flush.

[seliopou]

This is a pretty bad bug. And while creating a new buffer on every flush fixes the problem, it also defeats one of the main purposes of the library, which is under normal and well-tuned operation, the library shouldn't allocate large buffers.

I'm gonna merge this so that master is correct, and open an issue to solve the allocation problem in the near future.

Thanks again.