flvmux: Clear new_tags if sending metadata in header

This avoids sending an additional metadata object right after the
headers.
20 jobs for flvmux-tags in 62 minutes and 15 seconds (queued for 1 minute and 1 second)
Status Job ID Name Coverage
  Preparation
passed #378484
gst indent

00:00:49

passed #378483
manifest

00:00:45

 
  Build
passed #378490
build cerbero android universal

00:16:35

passed #378491
build cerbero cross win32

00:06:25

passed #378492
build cerbero cross win64

00:06:05

passed #378489
build cerbero fedora x86_64

00:07:16

manual #378494
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #378493
gst-macos-10.14
build cerbero macos x86_64

00:08:24

passed #378485
build fedora x86_64

00:06:06

passed #378486
build nodebug fedora x86_64

00:04:29

passed #378487
build static fedora x86_64

00:14:54

passed #378488
build static nodebug fedora x86_64

00:11:44

 
  Test
passed #380777
check fedora

00:03:39

passed #378496
integration testsuites fedora

00:11:55

passed #378498
valgrind bad

00:21:41

passed #378499
valgrind ges

00:09:28

passed #378497
valgrind good

00:20:07

failed #378495
check fedora

00:07:36

 
  Integrate
passed #378500
android universal examples

00:07:20

passed #378501
documentation

00:04:52