rtpmp4vpay: keep config data with interval -1

The payloader was stripping the configuration data when the
config-interval was set to 0. The code was written in such a way !(a >
0) that it stripped the config when it was set at -1 (send config_data
as soon as possible).

This resulted in some MPEG4 streams where no GOP/VOP-I was detected to
be sent out without configuration.
19 jobs for feature/send-mpeg4-config-at-idr in 76 minutes and 34 seconds (queued for 1 minute and 14 seconds)
Status Job ID Name Coverage
  Preparation
passed #432387
gst indent

00:00:39

passed #432386
manifest

00:00:32

 
  Build
passed #432393
build cerbero android universal

00:10:11

passed #432394
build cerbero cross win32

00:08:08

passed #432395
build cerbero cross win64

00:05:55

passed #432392
build cerbero fedora x86_64

00:06:43

manual #432397
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #432396
gst-macos-10.14
build cerbero macos x86_64

00:07:44

passed #432388
build fedora x86_64

00:06:21

passed #432389
build nodebug fedora x86_64

00:10:37

passed #432390
build static fedora x86_64

00:13:07

passed #432391
build static nodebug fedora x86_64

00:12:01

 
  Test
passed #432398
check fedora

00:03:47

passed #432399
integration testsuites fedora

00:26:46

passed #432401
valgrind bad

00:07:54

passed #432402
valgrind ges

00:07:27

passed #432400
valgrind good

00:22:52

 
  Integrate
passed #432403
android universal examples

00:08:29

passed #432404
documentation

00:04:40