res_rtp_asterisk: Fix DTLS when used with OpenSSL 1.0.1k

A recent security fix for OpenSSL broke DTLS negotiation for many
applications. This was caused by read ahead not being enabled when it
should be. While a commit has gone into OpenSSL to force read ahead
on for DTLS it may take some time for a release to be made and the
change to be present in distributions (if at all). As enabling read
ahead is a simple one line change this commit does that and fixes
the issue.

ASTERISK-24711 #close
Reported by: Jared Biel
........

Merged revisions 431384 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 431385 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@431386 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_rtp_asterisk.c

@@ -1281,6 +1281,8 @@ static int ast_rtp_dtls_set_configuration(struct ast_rtp_instance *instance, con
 		return -1;
 	}
 
+	SSL_CTX_set_read_ahead(rtp->ssl_ctx, 1);
+
 	rtp->dtls_verify = dtls_cfg->verify;
 
 	SSL_CTX_set_verify(rtp->ssl_ctx, (rtp->dtls_verify & AST_RTP_DTLS_VERIFY_FINGERPRINT) || (rtp->dtls_verify & AST_RTP_DTLS_VERIFY_CERTIFICATE) ?