lib-sieve: sieve-result - Fix panic occurring upon temp fail after committed keep-equivalent action.

The determination of whether a keep-equivalent action was already executed is
performed in sieve_result_implicit_keep_execute(), which can also be called from
sieve_result_implicit_keep_finalize(). But there, the keep-equivalent status was
checked before any call to sieve_result_implicit_keep_execute(), which would
thus yield the wrong result.

This change also makes the associated debug message more specific (otherwise two
identical debug messages are logged) and an assert is added that makes sure the
keep-equivalent action is actually finalized when the implicit keep was supposed
to be finalized.

Panic was:

Panic: file sieve-result.c: line 1706 (sieve_result_implicit_keep_finalize): assertion failed: (aexec_keep->state == SIEVE_ACTION_EXECUTION_STATE_EXECUTED)