Plane: re-enable geofencing on APM2

it only just fits ...

ArduPlane/config.h

@@ -420,11 +420,7 @@
 
 // use this to disable geo-fencing
 #ifndef GEOFENCE_ENABLED
-#if HAL_CPU_CLASS > HAL_CPU_CLASS_16
  # define GEOFENCE_ENABLED ENABLED
-#else
- # define GEOFENCE_ENABLED DISABLE
-#endif
 #endif
 
 // pwm value on FENCE_CHANNEL to use to enable fenced mode

ArduPlane/geofence.cpp

@@ -94,7 +94,8 @@ void Plane::geofence_load(void)
     uint8_t i;
 
     if (geofence_state == NULL) {
-        if (hal.util->available_memory() < 512 + sizeof(struct GeofenceState)) {
+        uint16_t boundary_size = sizeof(Vector2l) * max_fencepoints();
+        if (hal.util->available_memory() < 100 + boundary_size + sizeof(struct GeofenceState)) {
             // too risky to enable as we could run out of stack
             goto failed;
         }
@@ -104,7 +105,7 @@ void Plane::geofence_load(void)
             goto failed;
         }
 
-        geofence_state->boundary = (Vector2l *)calloc(sizeof(Vector2l), max_fencepoints());
+        geofence_state->boundary = (Vector2l *)calloc(1, boundary_size);
         if (geofence_state->boundary == NULL) {
             free(geofence_state);
             geofence_state = NULL;