Fix crash in krping when run as a client due to NULL pointer access.

Initialize pointer in question which is used only when fast registers
mode is selected.

Sponsored by:	Mellanox Technologies
MFC after:	1 week

sys/contrib/rdma/krping/krping.c

@@ -261,6 +261,7 @@ static int krping_cma_event_handler(struct rdma_cm_id *cma_id,
 
 	case RDMA_CM_EVENT_ROUTE_RESOLVED:
 		cb->state = ROUTE_RESOLVED;
+		cb->child_cm_id = cma_id;
 		wake_up_interruptible(&cb->sem);
 		break;