bug#12808: segfault with (weird use of) make-struct and make-vtable

[Areth Unknown]

This code makes guile segfault, tested in the versions 2.0.6.49 and 2.0.6.66 ..

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(define v-blocks (make-vtable "pR")) ;; note that this is a protected tail array read-only

(define block-2 (make-struct v-blocks 1 ;; note that this only has one element
                             '((0 2 2 0)
                               (0 2 2 0)
                               (0 0 0 0)
                               (0 0 0 0))))

;; (struct-ref block-2 0)

;; => Segmentation fault (core dumped)
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There isn't really any reason (for me) to use make-struct or make-vtable (instead of srfi-9) as they are very low level, but it still shouldn't segfault.

So now it's reported.

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// add