Our regression test has void* lists when we'd expected void**. This, however, went unnoticed as we only tried to access the first element. Attempts to access the second element would have been misaligned, and failed an invariant as of 35cc503 (we don't handle pointer arithmetic over void pointers in the back-end anymore).

We now make sure that we always increment by size-of-void*.

Fixes: #7706

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