bug#31606: Predicate instance? doesn't work for non-struct objects

[Tommi Höynälänmaa]

Predicate instance? in Guile 2.2.3 doesn't work for non-struct objects.

In order to reproduce the bug give the following commands in Guile:
---cut here---
(import (oop goops))
(instance? 1)
---cut here---

The actual output is:
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<unnamed port>:2:0: In procedure struct_vtable: Wrong type argument in 
position 1 (expecting struct): 1

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
---cut here---

I use Guile on Ubuntu 18.04.

     - Tommi Höynälänmaa