Prevent Swift objects' deinit from trying to free a nil pointer whe…

…n subclassed for mocking (#2001)

- doing so will result in a `Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value`
- deinit is not overridable in Swift and this cannot be handled in the subclass

uniffi_bindgen/src/bindings/swift/templates/ObjectTemplate.swift

@@ -63,6 +63,10 @@ open class {{ impl_class_name }}:
     {%- endmatch %}
 
     deinit {
+        guard let pointer = pointer else {
+            return
+        }
+
         try! rustCall { {{ obj.ffi_object_free().name() }}(pointer, $0) }
     }