The unit allocated for decNumberCompare was accidentally removed by
commit 680baeffeb (PR #2804)
This caused a stack overflow when comparing a nan with a payload of 1000
or more.
This bug was found by OSS-fuzz.
Ref: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=64771
Fixes GHSA-7hmr-442f-qc8j
It also fixes 1e999999999 > 1e-1147483646 triggering UBSAN errors
Fixes#2968