uri-asset: Ensure that the discoverer stops on deinit.

Discoverer maintain a referernce on the discoverer object while
the async timeout callback is alive to prevent a potential crash
if the object is freed while the callback is pending.

But if g_main_context is released before calling the timeout callback,
the discoverer pointer which was weak referenced from GESUriClipAssetClass
will not be disposed because the discoverer object is not finalized.
11 jobs for <span class="ref-name">fix-discoverer-stop</span> in 33 minutes and 56 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #237788
gst indent

00:00:20

passed #237787
manifest

00:00:43

 
  Build
passed #237789
build fedora x86_64

00:06:17

passed #237790
build nodebug fedora x86_64

00:04:56

 
  Test
passed #237791
check fedora

00:02:59

passed #237792
integration testsuites fedora

00:07:19

 
  Full Builds
passed #237794
build cerbero android universal

00:13:01

passed #237795
build cerbero cross win32

00:08:21

passed #237796
build cerbero cross win64

00:06:01

passed #237793
build cerbero fedora x86_64

00:06:58

 
  Apps
passed #237797
android universal examples

00:06:33