If `FlutterDartProject` found an `FLTLibraryPath` entry in an iOS
application's `Info.plist`, it assumed that values that were valid
filesystem paths were paths to bundles. If the attempt to retrieve
the `NSBundle` fails, `FlutterDartProject` ignored the failure and
then would assign `nil` to a C++ `std::string`, resulting in a null
pointer dereference.
Add some failure checks to prevent this.