[swift3] fix infinite loop with inheritance and check if parent decoder exists

[julienfouilhe]

PR checklist

• Read the contribution guidelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

My last commit (b1a39ac) had introduced an infinite loop if you were using discriminator and returning a class that has a parent in one of your routes. By ensuring the instance is nil before "going down" in the tree, we ensure that we're not already "going up", thus removing the infinite loop.

Steps to reproduce

In the petstore, add a route /cats returning Cat elements only. It will first go through the Cat decoder, that will call the Animal decoder, which has a discriminator on className and thus will call the Cat decoder again...

[julienfouilhe]

cc @wing328 @jaz-ah @Edubits @jgavris @hexelon