Fix use-after-free in GcalManager code

This was originally discovered and fixed by Milan Crha; I'm just
forward-porting his fix to the master branch. self->clients takes
ownership of this source but we don't currently have ownership here,
leading to a double free vulnerability.

Fixes #435
3 jobs for mcatanzaro/#435 in 4 minutes and 40 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Flatpak
passed #416765
flatpak

00:04:11

 
  Review
passed #416766
review

00:00:29

manual #416767
allowed to fail manual
stop_review