AP_Baro: prevent a race condition in the SITL baro driver

libraries/AP_Baro/AP_Baro_HIL.cpp

@@ -19,6 +19,7 @@ uint8_t AP_Baro_HIL::read()
     uint8_t result = 0;
 
     if (_count != 0) {
+        hal.scheduler->suspend_timer_procs();
         result = 1;
         Press = ((float)_pressure_sum) / _count;
         Temp = ((float)_temperature_sum) / _count;
@@ -26,6 +27,7 @@ uint8_t AP_Baro_HIL::read()
         _count = 0;
         _pressure_sum = 0;
         _temperature_sum = 0;
+        hal.scheduler->resume_timer_procs();
     }
 
     return result;

libraries/AP_Baro/AP_Baro_HIL.h

@@ -13,7 +13,7 @@ private:
     float Press;
     int32_t _pressure_sum;
     int32_t _temperature_sum;
-    uint8_t _count;
+    volatile uint8_t _count;
 
 public:
     bool init();