axis-vars: guard motor lookup for synthetic motor 4

_compute_axis indexed config.motors[motor_id] directly without
checking the array length. For motor_id=4 (the synthetic external-
axis motor used by ExternalAxis) there is no entry in config.motors,
so motor was undefined and motor["homing-mode"] threw - which
made the entire 'a' computed prop return undefined and the A row
never rendered.

Default to {} when the index is out of bounds.

src/js/axis-vars.js

@@ -56,7 +56,12 @@ module.exports = {
             const abs = this.state[`${axis}p`] || 0;
             const off = this.state[`offset_${axis}`];
             const motor_id = this._get_motor_id(axis);
-            const motor = motor_id == -1 ? {} : this.config.motors[motor_id];
+            // motor_id may be 4 for the synthetic external-axis motor;
+            // there is no entry for it in config.motors so guard with
+            // an empty object to avoid undefined property access.
+            const motor = (motor_id == -1
+                ? {}
+                : (this.config.motors[motor_id] || {}));
             const enabled = this._check_is_enabled(axis);
             const homingMode = motor["homing-mode"];
             const homed = this.state[`${motor_id}homed`];