Copter: fixed RC failsafe handling for no RC receiver

this stops us using uninitialised values in modes like circle which
can operate either with or without RC input. If we didn't have a RC
receiver attached then they would use a maximum yaw rate (which
produces quite a spectacular result for a tuned up racing quad)

ArduCopter/Attitude.cpp

@@ -4,6 +4,10 @@
 // returns desired yaw rate in centi-degrees per second
 float Copter::get_pilot_desired_yaw_rate(int16_t stick_angle)
 {
+    // throttle failsafe check
+    if (failsafe.radio || !ap.rc_receiver_present) {
+        return 0.0f;
+    }
     float yaw_request;
 
     // calculate yaw rate request
@@ -76,7 +80,7 @@ void Copter::set_throttle_takeoff()
 float Copter::get_pilot_desired_climb_rate(float throttle_control)
 {
     // throttle failsafe check
-    if (failsafe.radio) {
+    if (failsafe.radio || !ap.rc_receiver_present) {
         return 0.0f;
     }
 

ArduCopter/mode.cpp

@@ -351,6 +351,12 @@ void Mode::update_navigation()
 // returns desired angle in centi-degrees
 void Mode::get_pilot_desired_lean_angles(float &roll_out, float &pitch_out, float angle_max, float angle_limit) const
 {
+    // throttle failsafe check
+    if (copter.failsafe.radio || !copter.ap.rc_receiver_present) {
+        roll_out = 0;
+        pitch_out = 0;
+        return;
+    }
     // fetch roll and pitch inputs
     roll_out = channel_roll->get_control_in();
     pitch_out = channel_pitch->get_control_in();

ArduCopter/radio.cpp

@@ -94,11 +94,8 @@ void Copter::read_radio()
         set_throttle_and_failsafe(channel_throttle->get_radio_in());
         set_throttle_zero_flag(channel_throttle->get_control_in());
 
-        if (!ap.rc_receiver_present) {
+        // RC receiver must be attached if we've just got input
-            // RC receiver must be attached if we've just got input and
+        ap.rc_receiver_present = true;
-            // there are no overrides
-            ap.rc_receiver_present = !RC_Channels::has_active_overrides();
-        }
 
         // pass pilot input through to motors (used to allow wiggling servos while disarmed on heli, single, coax copters)
         radio_passthrough_to_motors();