Fixed infinite recursion when failing to download shield image

If there’s no image after the shield downloading operation completes, then there’s no reason to regenerate the attributed text that would embed that image.

CHANGELOG.md

@@ -14,6 +14,7 @@
 
 * The `NavigationSettings.shared` property is now accessible in Objective-C code as `MBNavigationSettings.sharedSettings`. ([#1882](https://github.com/mapbox/mapbox-navigation-ios/pull/1882))
 * Fixed spurious rerouting on multi-leg routes. ([#1884](https://github.com/mapbox/mapbox-navigation-ios/pull/1884))
+* Fixed a hang that occurred when failing to fetch a route shield image for display in a visual instruction banner. ([#1888](https://github.com/mapbox/mapbox-navigation-ios/pull/1888))
 * Adding property `RouteController.nearbyCoordinates`, which offers similar behavior to `RouteLegProgress.nearbyCoordinates`, which the addition of step lookahead/lookbehind in multi-leg routes. ([#1883](https://github.com/mapbox/mapbox-navigation-ios/pull/1883))
 * The `MGLShapeSourceOptions.lineDistanceMetrics` property has been temporarily disabled from the route line shape source due to a crash. This means it isn’t possible to set the `MGLLineStyleLayer.lineGradient` property on the route line style layers. ([#1886](https://github.com/mapbox/mapbox-navigation-ios/pull/1886))
 

MapboxNavigation/InstructionPresenter.swift

@@ -237,6 +237,7 @@ class InstructionPresenter {
     }
 
     private func completeShieldDownload(_ image: UIImage?) {
+        guard image != nil else { return }
         //We *must* be on main thread here, because attributedText() looks at object properties only accessible on main thread.
         DispatchQueue.main.async {
             self.onShieldDownload?(self.attributedText()) //FIXME: Can we work with the image directly?