Copter: fix underflow in scheduler

If fast_loop method executed time is over MAIN_LOOP_MICROS,
scheduler.run method set value is  0.

ArduCopter/ArduCopter.cpp

@@ -238,7 +238,7 @@ void Copter::loop()
     // the first call to the scheduler they won't run on a later
     // call until scheduler.tick() is called again
     uint32_t time_available = (timer + MAIN_LOOP_MICROS) - micros();
-    scheduler.run(time_available);
+    scheduler.run(time_available > MAIN_LOOP_MICROS ? 0u : time_available);
 }