When we miss one or more packets in stateful mode *and* need to perform a key change, *and* our sequence numbers have wrapped, ensure that the number of key changes is calculated correctly.
When we miss one or more packets in stateful mode *and* need to
perform a key change, *and* our sequence numbers have wrapped,
ensure that the number of key changes is calculated correctly.

The previous code counted down from a negative number to zero,
re-encrypting the current key on each iteration - this took some
time and strangely enough got the answer wrong !!!

Fix a(nother) spelling mistake while I'm there.

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