MRS-2850 Crash if listen/cancel is executed in quick succession

The onDidAuthorizationChanged is executed by iOS somewhen afer CLLocationManager is created in onListenWithArguments.

If the the subscription is canceled in the meantime _eventSink (onCancelWithArguments) is set to nil leading to accessing _eventSink when already nil (BAD_ACCESS crash).

Author: mkurz000

geolocator_apple/ios/Classes/Handlers/LocationServiceStreamHandler.m

@@ -37,10 +37,12 @@ - (void)locationManager:(CLLocationManager *)manager didChangeAuthorizationStatu
   dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
     BOOL isEnabled = [CLLocationManager locationServicesEnabled];
     dispatch_async(dispatch_get_main_queue(), ^(void) {
-        if (isEnabled) {
-            self->_eventSink([NSNumber numberWithInt:(ServiceStatus)enabled]);
-        } else {
-            self->_eventSink([NSNumber numberWithInt:(ServiceStatus)disabled]);
+        if (_eventSink != nil) {
+            if (isEnabled) {
+                self->_eventSink([NSNumber numberWithInt:(ServiceStatus) enabled]);
+            } else {
+                self->_eventSink([NSNumber numberWithInt:(ServiceStatus) disabled]);
+            }
         }
     });
   });