ns: Fix error return value in __get_ns_id()

Function has unsigned int return value, and error is coded by 0, not -1.

travis-ci: success for ns: Fix error return value in __get_ns_id()
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/namespaces.c

@@ -449,7 +449,7 @@ static unsigned int __get_ns_id(int pid, struct ns_desc *nd, protobuf_c_boolean
 			goto out;
 		}
 		pr_perror("Unable to stat %s", ns_path);
-		return -1;
+		return 0;
 	}
 	kid = st.st_ino;
 	BUG_ON(!kid);