panic in `convert_error` for unreasonably long lines

[matt-phylum]

One of my coworkers found that since rust-lang/rust#136932 (Rust 1.87), there is a footgun where even though you can pass a usize number of characters to fmt, there is a an upper limit of 0xffff, and anything higher than that results in a panic. This affects nom's convert_error function when an error is found more than 0xffff bytes from the start of the line, because convert_error will try output that many spaces using the format specifier.