libosmo-mgcp-client: fix use-after-free in mgcp_msg_gen()

Change-Id: Ib8b6c25489a6a704912aa1763d7430c8055d54e3
diff --git a/src/libosmo-mgcp-client/mgcp_client.c b/src/libosmo-mgcp-client/mgcp_client.c
index e9b3ec0..54db20d 100644
--- a/src/libosmo-mgcp-client/mgcp_client.c
+++ b/src/libosmo-mgcp-client/mgcp_client.c
@@ -1208,6 +1208,7 @@
 			     "Endpoint name (%s) lacks separator (@), can not generate MGCP message\n",
 			     mgcp_msg->endpoint);
 			msgb_free(msg);
+			return NULL;
 		}
 
 		rc += msgb_printf(msg, " %s", mgcp_msg->endpoint);