res_rtp_asterisk: Don't leak memory or reset state if DTLS configuration is set multiple times.
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Merged revisions 417705 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@417706 f38db490-d61c-443f-a65b-d21fe96a405b