[mgcp] The networking code needs a source addrs..

Make the source address mandantory and complain about
complain when it is missing. The address is mandantory
as it needs to be put into the MGCP messages...
diff --git a/openbsc/src/mgcp/mgcp_vty.c b/openbsc/src/mgcp/mgcp_vty.c
index ac8e873..6773d88 100644
--- a/openbsc/src/mgcp/mgcp_vty.c
+++ b/openbsc/src/mgcp/mgcp_vty.c
@@ -274,6 +274,11 @@
 	if (!g_cfg->bts_ip)
 		fprintf(stderr, "No BTS ip address specified. This will allow everyone to connect.\n");
 
+	if (!g_cfg->source_addr) {
+		fprintf(stderr, "You need to specify a bind address.\n");
+		return -1;
+	}
+
 	if (mgcp_endpoints_allocate(g_cfg) != 0) {
 		fprintf(stderr, "Failed to allocate endpoints: %d. Quitting.\n", g_cfg->number_endpoints);
 		return -1;