Allow the refcount to be > 0 after GSource::finalize() has returned

This can happen if weak references are emulated via the finalize
function, as happens in GStreamer's GstBus and in GIO's GCancellable.
7 jobs for source-atomic-refcounting in 13 minutes and 51 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #227045
cross-android_api21_arm64

00:03:13

passed #227046
cross-android_api28_arm64

00:03:10

passed #227047
cross-mingw64

00:03:25

passed #227044
fedora-x86_64

00:05:34

passed #227048
win32
msys2-mingw32

00:11:18

passed #227049
win32
vs2017-x64

00:08:16

 
  Coverage
passed #227050
coverage

00:01:59

74.3%