Fix add with overflow panic

[rgreinho]

While working on adding a new example to showcase the use of indicatif
with async multi-progress bars, I faced a random panic bug:

thread 'main' panicked at 'attempt to add with overflow', src/state.rs:424:40

As a result this patch provides a new example and a fix for a bug it
exposed.

[rgreinho]

To reproduce the issue without the patch, run the example several times in a row:

export RUST_BACKTRACE=1
for i in {0..9};do echo "---Run #${i---}"; cargo run -q --example async-multi-main; done

Which would output something like that at some point:

---Run #7
██████████░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░       3/12
██████████████████████████████████░░░░░░     813/942
████████████████████████████████████░░░░     884/980
████████████████████████████░░░░░░░░░░░░     193/274
█████████████████░░░░░░░░░░░░░░░░░░░░░░░     124/283                                                                 thread 'main' panicked at 'attempt to add with overflow', src/state.rs:424:40
stack backtrace:
   0: rust_begin_unwind
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/std/src/panicking.rs:498:5
   1: core::panicking::panic_fmt
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/panicking.rs:116:14
   2: core::panicking::panic
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/panicking.rs:48:5
   3: indicatif::state::AtomicPosition::allow
             at ./src/state.rs:424:40
   4: indicatif::progress_bar::ProgressBar::inc
             at ./src/progress_bar.rs:156:12
   5: morebars_multi::add_bar::{{closure}}
             at ./examples/morebars-multi.rs:59:5
   6: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/future/mod.rs:84:19
   7: <futures_util::stream::futures_unordered::FuturesUnordered<Fut> as futures_core::stream::Stream>::poll_next
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.21/src/stream/futures_unordered/mod.rs:514:17
   8: futures_util::stream::stream::StreamExt::poll_next_unpin
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.21/src/stream/stream/mod.rs:1626:9
   9: <futures_util::stream::stream::buffer_unordered::BufferUnordered<St> as futures_core::stream::Stream>::poll_next
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.21/src/stream/stream/buffer_unordered.rs:79:15
  10: <futures_util::stream::stream::collect::Collect<St,C> as core::future::future::Future>::poll
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.21/src/stream/stream/collect.rs:50:26
  11: morebars_multi::main::{{closure}}
             at ./examples/morebars-multi.rs:37:29
  12: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/future/mod.rs:84:19
  13: tokio::park::thread::CachedParkThread::block_on::{{closure}}
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.17.0/src/park/thread.rs:263:54
  14: tokio::coop::with_budget::{{closure}}
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.17.0/src/coop.rs:102:9
  15: std::thread::local::LocalKey<T>::try_with
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/std/src/thread/local.rs:412:16
  16: std::thread::local::LocalKey<T>::with
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/std/src/thread/local.rs:388:9
  17: tokio::coop::with_budget
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.17.0/src/coop.rs:95:5
  18: tokio::coop::budget
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.17.0/src/coop.rs:72:5
  19: tokio::park::thread::CachedParkThread::block_on
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.17.0/src/park/thread.rs:263:31
  20: tokio::runtime::enter::Enter::block_on
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.17.0/src/runtime/enter.rs:151:13
  21: tokio::runtime::thread_pool::ThreadPool::block_on
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.17.0/src/runtime/thread_pool/mod.rs:73:9
  22: tokio::runtime::Runtime::block_on
             at /Users/rgreinhofer/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.17.0/src/runtime/mod.rs:477:43
  23: morebars_multi::main

The same command runs without error one the fix is in.

[djc]

I think #406 is a better fix, we'll likely merge that soon.

[rgreinho]

@djc Would you still accept another PR with my example?

[djc]

I think indicatif already has too many examples -- I'd prefer that you modify one of the existing examples to cover additional functionality if you think some functionality is not covered well enough.

And -- forgot to say this the first time around, sorry -- thanks for contributing feedback and code!