Steps to reproduce:
1. Open any project
2. Create any Object in the Hierarchy window and name so that it has either "/" or "\" in its name
3. Open the Animation window and try creating a new animation while having the Object selected

Expected: the Animation and an Animator is created
Actual: the Animator cannot be created and an error is thrown, the Animation does get created

Reproduced in: 5.6.6f1, 2017.1.3p3, 2017.2.2p3, 2017.4.1f1, 2018.1.0f1, 2018.2.0b1

Error: Invalid AssetDatabase path : /.controller. use path relative to the project folder.
in 2018.1.0f1: Couldn't create asset file! UnityException: Creating asset at path /Cube.controller failed.