olm: Also initialize all fields when decoding Olm messages.

As a precaution.
5 jobs for fix-decoding-use-of-uninit-value in 2 minutes and 5 seconds (queued for 17 seconds)
Status Job ID Name Coverage
  Build
passed build:lib #13718
docker

00:01:15

 
  Test
passed test:lib #13719
docker

00:00:49

 
  Trigger
passed trigger:android

passed trigger:javascript

passed trigger:python