AP_L1_Control: Sanatize loiter radius to prevent bad input from

propegating

libraries/AP_L1_Control/AP_L1_Control.cpp

@@ -336,7 +336,7 @@ void AP_L1_Control::update_loiter(const struct Location &center_WP, float radius
 
     // scale loiter radius with square of EAS2TAS to allow us to stay
     // stable at high altitude
-    radius = loiter_radius(radius);
+    radius = loiter_radius(fabsf(radius));
 
     // Calculate guidance gains used by PD loop (used during circle tracking)
     float omega = (6.2832f / _L1_period);