rtsp-client: RTP Info exists conditionally in PLAY

If RTP Info is missing and it is not a receiver only, eg. audio
backchannel. Then return GST_RTSP_STS_INTERNAL_SERVER_ERROR.
In rfc2326 it says RTP-info is req. but in RFC7826 it is conditional.

Since 1.14 there is audio backchannel support. Thus RTP-info is
conditional now. When audio backchannel only mode, there is no RTP-info.

Fixes #82
Status Job ID Name Coverage
  Preparation
passed #650348
gst indent

00:00:20

passed #650347
manifest

00:00:33

 
  Build
passed #651657
build cerbero cross-android universal

00:12:07

passed #651660
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal

00:41:04

passed #650359
build cerbero cross win32

00:10:43

passed #650360
build cerbero cross win64

00:07:42

passed #650357
build cerbero fedora x86_64

00:42:30

passed #651658
gst-macos-10.14
build cerbero macos x86_64

00:09:59

passed #650353
build clang fedora x86_64

00:06:42

passed #650349
build fedora x86_64

00:06:24

passed #650707
docker windows 1607
build msys2

00:14:10

passed #650350
build nodebug fedora x86_64

00:05:04

passed #650351
build static fedora x86_64

00:12:57

passed #650352
build static nodebug fedora x86_64

00:03:29

passed #650354
docker windows 1607
build vs2017 amd64

00:11:11

passed #651659
docker windows 1607
build vs2017 x86

00:11:52

failed #650358
build cerbero cross-android universal

01:00:00

failed #650362
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal

01:00:00

failed #650361
gst-macos-10.14
build cerbero macos x86_64

01:00:00

failed #650356
docker windows 1607
build msys2

00:00:23

failed #650355
docker windows 1607
build vs2017 x86

01:19:27

 
  Test
passed #650363
check fedora

00:03:57

passed #650364
integration testsuites fedora

00:13:09

 
  Integrate
passed #650365
cross-android universal examples

00:07:36

manual #650367
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #650366
documentation

00:05:32