rtsp-media: Fix race codition in finish_unprepare

The previous fix for race condition around finish_unprepare where the
function could be called twice assumed that the status wouldn't change
during execution of the function. This assumption is incorrect as the
state may change, for example if an error message arrives from the
pipeline bus.

Instead a flag keeping track on whether the finish_unprepare function
is currently executing is introduced and checked.

Fixes #1

Change-Id: I7f5a4182a3f64f350b9d6ac3fd324475f248c0b6
6 jobs for master in 28 minutes and 20 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #69872
gst indent

00:00:21

passed #69871
manifest

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  Build
passed #69873
build fedora x86_64

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  Test
passed #69874
check fedora

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  Full Builds
passed #69875
build android arm64 api28

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passed #69876
build cerbero fedora x86_64

00:07:46