Close libsrtp properly

Ensure that libsrtp is shutdown properly when res_srtp is unloaded.

(closes issue ASTERISK-21719)
Reported by: Corey Farrell
Patches:
    res_srtp-library-shutdown.patch uploaded by Corey Farrell
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Merged revisions 388529 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 388530 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@388531 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_srtp.c

@@ -546,6 +546,7 @@ static void res_srtp_shutdown(void)
 {
 	srtp_install_event_handler(NULL);
 	ast_rtp_engine_unregister_srtp();
+	srtp_shutdown();
 	g_initialized = 0;
 }