fix: Avoid capacity leak or panic when a stream is cancelled after reserve_capacity

[ArniDagur]

The following triggers a bug

let (resp, mut send) = client.send_request(req, false)?;
send.reserve_capacity(N);
send.send_reset(Reason::CANCEL);

In builds with debug assertions enabled, we panic:

thread 'reserve_capacity_then_cancel_does_not_leak' panicked at src/proto/streams/prioritize.rs:445:9:
state=Closed(Error(Reset(StreamId(1), CANCEL, User)))
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: h2::proto::streams::prioritize::Prioritize::try_assign_capacity
             at src/proto/streams/prioritize.rs:445:9
   3: h2::proto::streams::prioritize::Prioritize::poll_complete
             at src/proto/streams/prioritize.rs:535:22
   4: h2::proto::streams::send::Send::poll_complete
             at src/proto/streams/send.rs:350:14
   5: h2::proto::streams::streams::Inner::poll_complete
             at src/proto/streams/streams.rs:906:34
   6: h2::proto::streams::streams::Streams<B,P>::poll_complete
             at src/proto/streams/streams.rs:186:12
   7: h2::proto::connection::Connection<T,P,B>::poll
             at src/proto/connection.rs:287:55
   8: <h2::client::Connection<T,B> as core::future::future::Future>::poll
             at src/client.rs:1441:33

In production builds, it instead leaks capacity, since the assert is compiled out.

[seanmonstar]

What do you think, worth a trace, or is it not interesting enough?

[ArniDagur]

I don't have a strong opinion here, but I lean towards not adding a trace.

1. The existing guard for the same condition at another location does not log
2. I'm not sure what the actionable item would be for someone that sees the log