Bug 1095914 - Fix crash if GPS not available. r=vng

https://github.com/mozilla/MozStumbler/pull/1206

mobile/android/stumbler/java/org/mozilla/mozstumbler/service/stumblerthread/scanners/GPSScanner.java

@@ -65,8 +65,23 @@ public class GPSScanner implements LocationListener {
         }
     }
 
+    private boolean isGpsAvailable(LocationManager locationManager) {
+        if (locationManager == null ||
+            locationManager.getProvider(LocationManager.GPS_PROVIDER) == null) {
+            String msg = "No GPS available, scanning not started.";
+            Log.d(LOG_TAG, msg);
+            AppGlobals.guiLogError(msg);
+            return false;
+        }
+        return true;
+    }
+
     private void startPassiveMode() {
         LocationManager locationManager = (LocationManager) mContext.getSystemService(Context.LOCATION_SERVICE);
+        if (!isGpsAvailable(locationManager)) {
+            return;
+        }
+
         locationManager.requestLocationUpdates(LocationManager.PASSIVE_PROVIDER,
                                                0,
                                                0, this);
@@ -74,6 +89,10 @@ public class GPSScanner implements LocationListener {
 
     private void startActiveMode() {
         LocationManager lm = getLocationManager();
+        if (!isGpsAvailable(lm)) {
+            return;
+        }
+
         lm.requestLocationUpdates(LocationManager.GPS_PROVIDER,
                                   ACTIVE_MODE_GPS_MIN_UPDATE_TIME_MS,
                                   ACTIVE_MODE_GPS_MIN_UPDATE_DISTANCE_M,