the bug fix was imported from 4.5.1, but it is important enough
to be highlighted in its own right.

I found out that the previous segfault fix did not correct the root
cause of the problem. Thus, I can re-instantiate the more performance-
optimal logic. In the next step, I'll merge in the real fix, so do NOT
use this commit as code you actually run!

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.