Fix comment about undefined behavior of constant_time_msb
Fix comment about undefined behavior of constant_time_msb

This comment was correct for the original commit introducing this
function (5a3d21c0585064292bde5cd34089e120487ab687), but was fixed
in commit d2fa182988afa33d9e950358de406cc9fb36d000 (and
67b8bcee95f225a07216700786b538bb98d63cfe)

Reviewed-by: Ben Kaduk <b34e9dc3dd24827b45bb22e8b76cde1bcb413b0e@mit.edu>
GH: #4975

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl