bus: Make removing of signal/bus watches thread-safe

Between getting the GSource with the mutex and destroying it, something
else might've destroyed it already and we would have a dangling pointer.

Keep an additional reference just in case.
8 jobs for remove-watch-signal in 17 minutes and 7 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #118824
gst indent

00:00:39

passed #118823
manifest

00:00:39

 
  Build
passed #118825
build fedora x86_64

00:03:29

 
  Test
passed #118826
check fedora

00:03:17

 
  Full Builds
passed #118827
build android arm64 api28

00:03:32

passed #118829
build cerbero cross win32

00:09:40

passed #118830
build cerbero cross win64

00:07:46

passed #118828
build cerbero fedora x86_64

00:08:02