User special variable for (not)initialised state detection

_mapScrollView can be not nil. We should use special variable for initialised state detection.

example:
```obj
- (void)viewDidLoad {
    [super viewDidLoad];

    self.mapView.tileSource = [[RMMBTilesSource alloc] initWithTileSetResource:@"map" ofType:@"mbtiles"];
    self.mapView.zoom = 12;
    ... more initialisation
    self.mapView.adjustTilesForRetinaDisplay = YES; // after that _mapScrollView != nil
    ...
```