fix: prevent double-decref of event in envelope add functions

[jpnurmi]

Move item->event assignment after the payload allocation check in sentry__envelope_add_event and sentry__envelope_add_transaction. This fixes the inconsistency between the various failure paths that this accidentally took ownership whereas the others don't, making it unsafe to decref the event on failure at the call-site.

Previously, if sentry__jsonwriter_into_string failed, the event was already owned by the envelope item. Existing callers decref the event and free the envelope on failure, leading to a double-decref because sentry_envelope_free also decrefs item->event via item cleanup:

sentry_envelope_t *
sentry__prepare_event(const sentry_options_t *options, sentry_value_t event,
    sentry_uuid_t *event_id, bool invoke_before_send,
    sentry_scope_t *local_scope)
{
    sentry_envelope_t *envelope = NULL;
    // ...
    if (!envelope || !sentry__envelope_add_event(envelope, event)) {
        goto fail;
    }
    // ...
    return envelope;

fail:
    sentry_envelope_free(envelope);
    sentry_value_decref(event); // <==
    return NULL;
}

Resolves #1493 (comment)