HAL_VRBrain: cope with nested cork/push

libraries/AP_HAL_VRBRAIN/RCOutput.cpp

@@ -531,6 +531,9 @@ void VRBRAINRCOutput::cork()
 
 void VRBRAINRCOutput::push()
 {
+    if (!_corking) {
+        return;
+    }
 #if RCOUT_DEBUG_LATENCY
     hal.gpio->pinMode(55, HAL_GPIO_OUTPUT);
     hal.gpio->write(55, 0);