rtprawdepay: Don't get rid of the buffer pool on FLUSH_STOP

We expect there to be a pool as long as the caps are known and
FLUSH_STOP is not resetting the caps. Getting rid of the pool would
cause assertions.

Fixes gstreamer/gst-plugins-good#584
17 jobs for rtprawdepay-flush-pool in 170 minutes and 6 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #272499
gst indent

00:00:30

passed #272498
manifest

00:00:38

 
  Build
passed #272500
build fedora x86_64

00:05:40

passed #272501
build nodebug fedora x86_64

00:05:30

 
  Test
passed #272502
check fedora

00:03:27

passed #272503
integration testsuites fedora

00:08:04

 
  Full Builds
passed #272897
build cerbero android universal

01:30:54

passed #272506
build cerbero cross win32

00:38:41

passed #272507
build cerbero cross win64

00:37:07

passed #272504
build cerbero fedora x86_64

00:39:48

failed #272658
build cerbero android universal

01:00:00

failed #272546
build cerbero android universal

01:00:01

failed #272505
build cerbero android universal

01:00:01

 
  Full Tests
passed #272509
valgrind bad

00:12:26

passed #272510
valgrind ges

00:03:57

passed #272508
valgrind good

00:16:52

 
  Apps
passed #272511
android universal examples

00:08:06