Motivations:
It seems that slicing a buffer and using this slice to write to CTX will
decrease the initial refCnt to 0, while the original buffer is not yet
fully used (not empty).
Modifications:
As suggested in the ticket and tested, when the currentBuffer is sliced
since it will still be used later on, the currentBuffer is retained.
Add a test case for this issue.
Result:
The currentBuffer still has its correct refCnt when reaching the last
write (not sliced) of 1 and therefore will be released correctly.
The exception does no more occur.
This fix should be applied to all branches >= 4.0.
50315f28c2