util: print_safe: harden conversion helpers
util: print_safe: harden conversion helpers

Add missing `requires std::is_integral_v<Integral>` concept
and assert that we don't overrun the provided buffer.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar