Use auto for unique pointers

samples/xmpprint.cpp

@@ -23,8 +23,7 @@ int main(int argc, char** argv)
         return 1;
       }
 
-    Exiv2::Image::UniquePtr image = Exiv2::ImageFactory::open(argv[1]);
-    assert (image.get() != 0);
+    auto image = Exiv2::ImageFactory::open(argv[1]);
     image->readMetadata();
 
     Exiv2::XmpData &xmpData = image->xmpData();