In async write mode, we use modular arithmetic to index the output
buffer array. However, the counter variables accidently were signed,
thus resulting in negative indizes after integer overflow. That in turn
could lead to segfaults, but was depending on the memory layout of
the instance in question (which in turn depended on a number of
variables, like compile settings but also configuration). The counters
are now unsigned (as they always should have been) and so the dangling
mis-indexing does no longer happen. This bug potentially affected all
installations, even if only some may actually have seen a segfault.