[linux kernel patch] fixed possible 'rtbTable' underflow in FSE_normalizeCount()

There's a patch set to linux kernel for a possible underflow, there was no reaction so I'm posting it here for awareness. https://lore.kernel.org/all/20251211171950.852001-1-devsec@tpz.ru/

As we may need to sync the code it would be better to have it upstream first and then forward it to linux kernel. I can send a separate pull request with just this fix (to linux kernel) assuming that it will be also in the zstd upstream.