nat: Use the mgcp_config_alloc to have defaults initialized

Initialize the net_base properly and benefit from future
config changes but also reset certain defauls that we do
not want at the nat.
diff --git a/openbsc/src/nat/bsc_mgcp_utils.c b/openbsc/src/nat/bsc_mgcp_utils.c
index e8f7e24..f038342 100644
--- a/openbsc/src/nat/bsc_mgcp_utils.c
+++ b/openbsc/src/nat/bsc_mgcp_utils.c
@@ -533,11 +533,19 @@
 	}
 
 	/* some more MGCP config handling */
+	if (nat->mgcp_cfg->audio_name)
+		talloc_free(nat->mgcp_cfg->audio_name);
+	nat->mgcp_cfg->audio_name = NULL;
+
 	nat->mgcp_cfg->audio_payload = -1;
 	nat->mgcp_cfg->data = nat;
 	nat->mgcp_cfg->policy_cb = bsc_mgcp_policy_cb;
 	nat->mgcp_cfg->force_realloc = 1;
+
+	if (nat->mgcp_cfg->bts_ip)
+		talloc_free(nat->mgcp_cfg->bts_ip);
 	nat->mgcp_cfg->bts_ip = "";
+
 	nat->bsc_endpoints = talloc_zero_array(nat,
 					       struct bsc_endpoint,
 					       nat->mgcp_cfg->number_endpoints + 1);
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index e5c2c39..9cf3ba3 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -1120,7 +1120,7 @@
 		return -4;
 	}
 
-	nat->mgcp_cfg = talloc_zero(nat, struct mgcp_config);
+	nat->mgcp_cfg = mgcp_config_alloc();
 	if (!nat->mgcp_cfg) {
 		fprintf(stderr, "Failed to allocate MGCP cfg.\n");
 		return -5;