rtpsession: Don't try to return srtpenc/dec if not installed

https://bugs.freedesktop.org/show_bug.cgi?id=85566
rtp-discover-codecs: Make global access to blueprints thread-safe

The global variable list_codec_blueprintfs refcounted with codecs_list_ref
was not thread safe. This patch uses a global lock to make this code path
thread safe.

https://bugs.freedesktop.org/show_bug.cgi?id=85567
Fix clang warnings

https://bugs.freedesktop.org/show_bug.cgi?id=85565
nicestreamtransmitter: expose port range setting from libnice