usnic: ensure that the string is NULL-terminated

This was CID 1269666.

opal/mca/btl/usnic/btl_usnic_cagent.c

@@ -1134,7 +1134,7 @@ int opal_btl_usnic_connectivity_agent_init(void)
 
     memset(&address, 0, sizeof(struct sockaddr_un));
     address.sun_family = AF_UNIX;
-    strncpy(address.sun_path, ipc_filename, sizeof(address.sun_path));
+    strncpy(address.sun_path, ipc_filename, sizeof(address.sun_path) - 1);
 
     if (bind(ipc_accept_fd, (struct sockaddr *) &address,
              sizeof(struct sockaddr_un)) != 0) {