Further reduce dependence on -fwrapv semantics in jsonb.

Further reduce dependence on -fwrapv semantics in jsonb.

Commit 108d2adb9e missed updating a few places in the jsonb code
that rely on signed integer wrapping for correctness.  These can
also be fixed by using pg_abs_s32() to negate a signed integer
(that is known to be negative) for comparison with an unsigned
integer.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/bfff906f-300d-81ea-83b7-f2c93845e7f2%40gmail.com