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 from fix-client-close-crash in 18 minutes and 57 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #93188
gst indent

00:00:20

passed #93187
manifest

00:00:22

 
  Build
passed #93189
build fedora x86_64

00:03:52

 
  Test
passed #93190
check fedora

00:03:24

 
  Full Builds
passed #93191
build android arm64 api28

00:05:47

passed #93193
build cerbero cross win32

00:11:17

passed #93194
build cerbero cross win64

00:05:16

passed #93192
build cerbero fedora x86_64

00:10:10