rtpjitterbuffer: Check the exit condition after executing timers

The do_expected_timeout() function may release the JBUF_LOCK, so we need
to check if nothing wanted the timer thread to exit after this call.
The side effect was that we may endup going back into waiting for a timer
which will cause arbitrary delay on tear down (or deadlock when test
clock is used).

Fixes #653
Status Job ID Name Coverage
  Preparation
passed #949300
gst indent

00:00:25

passed #949299
manifest

00:00:29

 
  Build
passed #949310
build cerbero cross-android universal

00:11:34

manual #949314
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #949311
build cerbero cross win32

00:08:55

passed #949312
build cerbero cross win64

00:05:44

passed #949309
build cerbero fedora x86_64

00:06:37

passed #949313
gst-macos-10.14
build cerbero macos x86_64

00:13:52

passed #949305
build clang fedora x86_64

00:08:35

passed #949301
build fedora x86_64

00:07:29

passed #949308
docker windows 1607
build msys2

00:13:09

passed #949302
build nodebug fedora x86_64

00:06:20

passed #949303
build static fedora x86_64

00:11:34

passed #949304
build static nodebug fedora x86_64

00:06:20

passed #949306
docker windows 1607
build vs2017 amd64

00:10:56

passed #949307
docker windows 1607
build vs2017 x86

00:11:40

 
  Test
passed #949315
gstreamer
check fedora

00:03:30

passed #949316
gstreamer
integration testsuites fedora

00:39:55

passed #949318
gstreamer
valgrind bad

00:07:17

passed #949319
gstreamer
valgrind ges

00:07:06

passed #949317
gstreamer
valgrind good

00:10:47

 
  Integrate
passed #949320
cross-android universal examples

00:04:43

manual #949322
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #949321
documentation

00:03:47