Merge branch 'mjog/650-old-val-0-glib-critical' into 'gnome-3-36'

Geary.Util.Source: Fix GLib old_val > 0 critical

Closes #650 (closed)

See merge request !455 (merged)

(cherry picked from commit 19a655bc)

8c43288b Geary.Util.Logging.Source: Fix GLib old_val > 0 critical