AP_Gripper: servo gripper not valid unless aux channel assigned

libraries/AP_Gripper/AP_Gripper_Backend.h

@@ -35,7 +35,7 @@ public:
     virtual void release() = 0;
 
     // valid - returns true if the backend should be working
-    bool valid() const { return true; };
+    virtual bool valid() const { return true; };
 
     // released - returns true if currently in released position
     virtual bool released() const = 0;

libraries/AP_Gripper/AP_Gripper_Servo.cpp

@@ -55,3 +55,14 @@ bool AP_Gripper_Servo::grabbed() const
 
 // type-specific periodic updates:
 void AP_Gripper_Servo::update_gripper() { };
+
+bool AP_Gripper_Servo::valid() const
+{
+    if (!AP_Gripper_Backend::valid()) {
+        return false;
+    }
+    if (!RC_Channel_aux::function_assigned(RC_Channel_aux::k_gripper)) {
+        return false;
+    }
+    return true;
+}

libraries/AP_Gripper/AP_Gripper_Servo.h

@@ -36,6 +36,9 @@ public:
     // released - returns true if gripper in released state
     bool released() const override;
 
+    // valid - returns true if the backend should be working
+    bool valid() const;
+
 protected:
 
     // type-specific intiailisations: