lib-sieve: sieve-result - Fix resource leak occurring when implicit keep is...

lib-sieve: sieve-result - Fix resource leak occurring when implicit keep is executed before temporary failure at commit.

In the commit phase the implicit keep was never finalized, meaning that it was
not rolled back and thus not cleaned up properly. This leads to a memory leak
and a mailbox reference leak. This in turn causes an assert crash at the end
of delivery when the mail user is destroyed.