res_rtp_asterisk: Don't leak temporary key when enabling PFS.

A change recently went in which enabled perfect forward secrecy for
DTLS in res_rtp_asterisk. This was accomplished two different ways
depending on the availability of a feature in OpenSSL. The fallback
method created a temporary instance of a key but did not free it.
This change fixes that.

ASTERISK-25265

Change-Id: Iadc031b67a91410bbefb17ffb4218d615d051396

res/res_rtp_asterisk.c

@@ -1254,6 +1254,9 @@ static int ast_rtp_dtls_set_configuration(struct ast_rtp_instance *instance, con
 {
 	struct ast_rtp *rtp = ast_rtp_instance_get_data(instance);
 	int res;
+#ifndef HAVE_OPENSSL_ECDH_AUTO
+	EC_KEY *ecdh;
+#endif
 
 	if (!dtls_cfg->enabled) {
 		return 0;
@@ -1273,8 +1276,11 @@ static int ast_rtp_dtls_set_configuration(struct ast_rtp_instance *instance, con
 #ifdef HAVE_OPENSSL_ECDH_AUTO
 	SSL_CTX_set_ecdh_auto(rtp->ssl_ctx, 1);
 #else
-	SSL_CTX_set_tmp_ecdh(rtp->ssl_ctx,
-		EC_KEY_new_by_curve_name(NID_X9_62_prime256v1));
+	ecdh = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
+	if (ecdh) {
+		SSL_CTX_set_tmp_ecdh(rtp->ssl_ctx, ecdh);
+		EC_KEY_free(ecdh);
+	}
 #endif
 
 	rtp->dtls_verify = dtls_cfg->verify;