MFC r316766:
Correct an out of bounds read with HN_AUTOSCALE and very large numbers.
The maximum scale is 6 (K, M, G, T, P, E) (B is 0).
Overly large explict scales were checked correctly, but for sufficently
large numbers HN_AUTOSCALE would get to 7 resulting in an out of bounds
read.
Found with humanize_number_test and CHERI bounds checking.
Reviewed by: emaste
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
git-svn-id: svn://svn.freebsd.org/base/stable/10@317387
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