subtitleoverlay: avoid send flush start and stop when sutitle track switching

when swith subtitle track, playsink will send flash start and stop event
we don't need send this event if the subtitle sink is not linked since
subtitle overlay is still in recreating and not ready
26 jobs for subtitle_error in 39 minutes and 23 seconds (queued for 1 minute and 21 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #610869
gst indent

00:01:24

passed #610868
manifest

00:00:51

 
  Build
passed #610879
build cerbero cross-android universal

00:14:33

manual #610883
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #610880
build cerbero cross win32

00:12:55

passed #610881
build cerbero cross win64

00:06:03

passed #610878
build cerbero fedora x86_64

00:06:48

passed #610882
gst-macos-10.14
build cerbero macos x86_64

00:09:50

passed #610874
build clang fedora x86_64

00:07:05

passed #610870
build fedora x86_64

00:08:50

passed #610877
docker windows 1607
build msys2

00:12:00

passed #610871
build nodebug fedora x86_64

00:04:55

passed #610872
build static fedora x86_64

00:13:56

passed #610873
build static nodebug fedora x86_64

00:03:23

passed #610875
docker windows 1607
build vs2017 amd64

00:10:12

passed #610876
docker windows 1607
build vs2017 x86

00:10:37

 
  Test
passed #610884
check fedora

00:04:11

failed #610885
integration testsuites fedora

00:17:13

passed #610889
valgrind bad

00:11:20

passed #610886
valgrind base

00:11:02

passed #610890
valgrind ges

00:10:24

passed #610887
valgrind good

00:12:32

passed #610888
valgrind ugly

00:01:55

 
  Integrate
passed #610891
cross-android universal examples

00:07:44

skipped #610893
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #610892
documentation

00:04:25

 
Name Stage Failure
failed
integration testsuites fedora Test
           Total: 1451
Uploading artifacts...
gst-build/build/meson-logs/: found 2 matching files
validate-logs: found 70065 matching files

Uploading artifacts to coordinator... ok
id=610885 responseStatus=201 Created token=iVhoq_qM
Uploading artifacts...
validate-logs/*.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=610885 responseStatus=201 Created token=iVhoq_qM
ERROR: Job failed: exit code 1