AP_NavEKF: Correctly report position timeout when GPS is lost

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -1928,7 +1928,10 @@ void NavEKF::FuseVelPosNED()
             // use position data if healthy, timed out, or in constant position mode
             if (posHealth || posTimeout || constPosMode) {
                 posHealth = true;
-                posFailTime = imuSampleTime_ms;
+                // We don't reset the failed time if we are in constant position mode
+                if (!constPosMode) {
+                    posFailTime = imuSampleTime_ms;
+                }
                 // if timed out or outside the specified glitch radius, increment the offset applied to GPS data to compensate for large GPS position jumps
                 if (posTimeout || (maxPosInnov2 > sq(float(_gpsGlitchRadiusMax)))) {
                     gpsPosGlitchOffsetNE.x += posInnov[0];