Fix panic when symbols are present in wikilink before pipe - #1004

[frostu8@SHAFT pulldown-cmark]$ echo "[[Wiki<|Link]]" | cargo run -- -W
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.22s
     Running `/home/frostu8/pulldown-cmark/target/debug/pulldown-cmark -W`
thread 'main' panicked at pulldown-cmark/src/parse.rs:2270:57:
begin <= end (8 <= 7) when slicing `[[Wiki<|Link]]
`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
<p><a href="Wiki%3C">