client: Fix crash in close handler

The close handler could trigger a crash because it invalidated the
watch_context while still leaving a source attached to it which would be
cleaned up at a later point.
8 jobs for fix-client-close-crash in 20 minutes and 32 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #92569
gst indent

00:00:28

passed #92568
manifest

00:00:44

 
  Build
passed #92570
build fedora x86_64

00:03:52

 
  Test
passed #92571
check fedora

00:05:11

 
  Full Builds
passed #92572
build android arm64 api28

00:05:04

passed #92574
build cerbero cross win32

00:09:12

passed #92575
build cerbero cross win64

00:07:31

passed #92573
build cerbero fedora x86_64

00:09:43