Fix null race-condition for heart rate sensor data

The previous implementation was technically safe because the sensor
never had null values. Now that it can, the calls to getHeartRate
must be stored instead of repeated to avoid a race condition where the
sensor data become null between the null-check and the second call

src/main/java/de/dennisguse/opentracks/sensors/sensorData/SensorDataSet.java

@@ -161,8 +161,9 @@ public class SensorDataSet {
             trackPoint.setPosition(gps.getAggregatedValue(trackPointCreator.createNow()));
         }
 
-        if (getHeartRate() != null) {
-            trackPoint.setHeartRate(getHeartRate().first);
+        Pair<HeartRate, String> heartRate = getHeartRate();
+        if (heartRate != null) {
+            trackPoint.setHeartRate(heartRate.first);
         }
 
         if (getCadence() != null) {