seq: format_float_shortest panic when printing absurdly large exponents

While playing with inputs at the edge of maximum possible exponents:

RUST_BACKTRACE=1 cargo run -p uu_seq -- -f "%g" 1e9223372036854775806 1e9223372036854775806 inf
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.18s
     Running `target/debug/seq -f '%g' 1e9223372036854775806 1e9223372036854775806 inf`

thread 'main' panicked at src/uucore/src/lib/features/format/num_format.rs:467:20:
attempt to add with overflow
stack backtrace:
   3: uucore::features::format::num_format::format_float_shortest
             at ./src/uucore/src/lib/features/format/num_format.rs:467:20
   4: <uucore::features::format::num_format::Float as uucore::features::format::num_format::Formatter<&uucore::features::format::extendedbigdecimal::ExtendedBigDecimal>>::fmt
             at ./src/uucore/src/lib/features/format/num_format.rs:278:21
   5: uucore::features::format::Format<F,T>::fmt
             at ./src/uucore/src/lib/features/format/mod.rs:383:9
   6: uu_seq::print_seq
             at ./src/uu/seq/src/seq.rs:265:9
   7: uu_seq::uumain::uumain
             at ./src/uu/seq/src/seq.rs:185:18
   8: uu_seq::uumain
             at ./src/uu/seq/src/seq.rs:88:1
   9: seq::main
             at ./src/uucore/src/lib/lib.rs:157:24
  10: core::ops::function::FnOnce::call_once
             at /home/drinkcat/nobackup/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
1e+9223372036854775806
2e+9223372036854775806
3e+9223372036854775806
4e+9223372036854775806
5e+9223372036854775806
6e+9223372036854775806
7e+9223372036854775806
8e+9223372036854775806
9e+9223372036854775806

%e has similar problems:

RUST_BACKTRACE=1 cargo run -p uu_seq -- -f "%e" 1e9223372036854775807 1e9223372036854775807 inf
thread 'main' panicked at src/uucore/src/lib/features/format/num_format.rs:410:20:
attempt to add with overflow
stack backtrace:
...
   3: uucore::features::format::num_format::format_float_scientific

Start in src/uucore/src/lib/features/format/num_format.rs, focusing on format_float_shortest and format_float_scientific, then reproduce the panic with the cargo run commands shown for uu_seq. Done means the equivalent absurd-exponent inputs no longer panic when seq formats them with %g or %e.