panic upon committing a zero-sized grant

[Vadim Kaushan]

Sometimes zero-sized grant points beyond the buffer size (grant_ptr=buffer_ptr+buffer_len), with such grant commit panics on assert!(self.is_our_grant(&grant.buf));.

[Jonas Schievink]

Repro:

use bbqueue::*;

fn main() {
    let bbq = bbq!(1000).unwrap();

    let size = 1000;
    let grant = bbq.grant(size).unwrap();
    bbq.commit(size, grant);

    let grant = bbq.grant(0).unwrap();
    bbq.commit(0, grant);
}

[James Munns]

I'd be open for a PR that fixes the check in is_our_grant!