lib-sieve: sieve-result - Fix omission of action finalization happening upon...

lib-sieve: sieve-result - Fix omission of action finalization happening upon result execution failure.

This occurred because the code that checks for a deferred keep action in
sieve_result_implicit_keep_execute() could end up "finding" an action that
wasn't actually a deferred keep. That action was then erroneously promoted to
the FINALIZED state, which meant that commit/rollback was never executed.