/// /// confuses rustfmt into a crush

[WaffleLapkin]

The following code crushes rustfmt with the format code in documentation comments option:

/// /// ```
/// ```
/// 
/// ```
/// ```
fn f() {}

; rustfmt --config format_code_in_doc_comments=true t.rs
thread 'main' panicked at 'begin <= end (12 <= 11) when slicing `fn main() {}
`', src/tools/rustfmt/src/lib.rs:399:47
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
; rustfmt --version
rustfmt 1.4.38-stable (7737e0b 2022-04-04)

[ytmimi]

Thanks for the report!

I'm going to close this as I believe this is a duplicate of both #5234 and #4793, and the issue is the empty code block.