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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@388529 f38db490-d61c-443f-a65b-d21fe96a405b