qsort: fix leonardo heap corruption from bug in doubleword ctz primitive

the pntz function, implementing a "count trailing zeros" variant for a
bit vector consisting of two size_t words, erroneously returned zero
rather than the number of bits in the low word when the first bit set
was the low bit of the high word.

as a result, a loop in the trinkle function which should have a
guaranteed small bound on the number of iterations, could run
unboundedly, thereby overflowing a stack-based working-space array
which was sized for the bound.

CVE-2026-40200 has been assigned for this issue.