AP_NavEKF2: Add protection against bad GPS time delay values

Prevent bad values for GPS time delay pushing the GPS time stamp outside the range of IMU data contained in the buffer. If this occurs it can prevent the GPS measurements from being fused and cause loss of navigation.

libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp

@@ -417,8 +417,8 @@ void NavEKF3_core::readGpsData()
             // Correct for the average intersampling delay due to the filter updaterate
             gpsDataNew.time_ms -= localFilterTimeStep_ms/2;
 
-            // Prevent time delay exceeding age of oldest IMU data in the buffer
-            gpsDataNew.time_ms = MAX(gpsDataNew.time_ms,imuDataDelayed.time_ms);
+            // Prevent the time stamp falling outside the oldest and newest IMU data in the buffer
+            gpsDataNew.time_ms = MIN(MAX(gpsDataNew.time_ms,imuDataDelayed.time_ms),imuDataDownSampledNew.time_ms);
 
             // Get which GPS we are using for position information
             gpsDataNew.sensor_idx = _ahrs->get_gps().primary_sensor();