mgcp_client: Check for osmo_fsm_register() error return value

Change-Id: Ie5e7fa419117349d098b0158ed840341094f3c16
diff --git a/src/libosmo-mgcp-client/mgcp_client_fsm.c b/src/libosmo-mgcp-client/mgcp_client_fsm.c
index 800ae1b..e910586 100644
--- a/src/libosmo-mgcp-client/mgcp_client_fsm.c
+++ b/src/libosmo-mgcp-client/mgcp_client_fsm.c
@@ -620,7 +620,8 @@
 
 	/* Register the fsm description (if not already done) */
 	if (fsm_registered == false) {
-		osmo_fsm_register(&fsm_mgcp_client);
+		if (osmo_fsm_register(&fsm_mgcp_client) < 0)
+			return NULL;
 		fsm_registered = true;
 	}