olm: Also initialize all fields when decoding Olm messages.

As a precaution.
5 jobs for master in 2 minutes and 32 seconds (queued for 24 seconds)
Status Name Job ID Coverage
  Build
passed build:lib #14067
docker

00:01:39

 
  Test
passed test:lib #14068
docker

00:00:52

 
  Trigger
passed trigger:android

passed trigger:javascript

passed trigger:python