Assert correct VM page passed to pruning

Before pruning a heap page, we get the current status of the
corresponding VM page. If the passed in vmbuffer isn't the right one,
visibilitymap_get_status() will silently unpin it and pin the correct
page. Pruning assumes the caller manages the vmbuffer lifecycle, so this
would leave the caller with a stale VM reference and would leak the new
VM pin. To avoid mistakes in development, assert that the correct VM
page is pinned before beginning.

Reported-by: Melanie Plageman <[email protected]>
Reviewed-by: Andrey Borodin <[email protected]>
Discussion: 
https://postgr.es/m/CAAKRu_amj7qLF4c=9ijd=708Fu2G8gg-2EqwBu=acdahu2s...@mail.gmail.com
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/40cc0662ed7c0bf07e8c6534b1291ec5bcb5fee0

Modified Files
--------------
src/backend/access/heap/pruneheap.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

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