rtsp-stream: Use cached address when allocating sockets

If an address/port was previously decided upon (ex: multicast in the
SDP), then use that instead of re-creating another one

Fixes gstreamer/gst-rtsp-server#57
8 jobs for <span class="ref-name">address-reuse</span> in 19 minutes and 39 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #92557
gst indent

00:00:17

passed #92556
manifest

00:00:21

 
  Build
passed #92558
build fedora x86_64

00:03:31

 
  Test
passed #92559
check fedora

00:03:51

 
  Full Builds
passed #92560
build android arm64 api28

00:03:30

passed #92562
build cerbero cross win32

00:09:51

passed #92563
build cerbero cross win64

00:07:21

passed #92561
build cerbero fedora x86_64

00:11:37