Fuzz crash on ```0{class=0#. when using goldmark-highlighting extension

[lmas]

Please answer the following before submitting your issue:

1. What version of goldmark are you using? : v1.3.5
2. What version of Go are you using? : go1.16
3. What operating system and processor architecture are you using? : linux/amd64
4. What did you do? : ran go-fuzz on my own project that uses the goldmark parser
5. What did you expect to see? : no crashes by goldmark
6. What did you see instead? :

panic: interface conversion: interface {} is float64, not []uint8

goroutine 1 [running]:
github.com/yuin/goldmark/parser.ParseAttributes.func1(0x7bede0, 0xdcfe00, 0x5, 0x5af701)
        ./.go/pkg/mod/github.com/yuin/goldmark@v1.3.3/parser/attribute.go:68 +0x2bf
github.com/yuin/goldmark/parser.Attributes.findUpdate(0xc00062d130, 0x2, 0x2, 0xdc2074, 0x5, 0x5, 0xc00047f8b0, 0x1)
        ./.go/pkg/mod/github.com/yuin/goldmark@v1.3.3/parser/attribute.go:37 +0xb7
github.com/yuin/goldmark/parser.ParseAttributes(0xc14930, 0xc00060a310, 0xc0bb64, 0x0, 0x412e82, 0xc00047f998)
        ./.go/pkg/mod/github.com/yuin/goldmark@v1.3.3/parser/attribute.go:67 +0x449
github.com/yuin/goldmark-highlighting.getAttributes(0xc000692000, 0xc0005876d3, 0xb, 0xd, 0xc0005876d3, 0xb)
        ./.go/pkg/mod/github.com/yuin/goldmark-highlighting@v0.0.0-20200307114337-60d527fdb691/highlighting.go:368 +0x148
github.com/yuin/goldmark-highlighting.(*HTMLRenderer).renderFencedCodeBlock(0xc00060a1c0, 0xc14188, 0xc00059d880, 0xc0005876d0, 0xe, 0x10, 0xc1bf80, 0xc000692000, 0x1, 0xdc98e0, ...)
        ./.go/pkg/mod/github.com/yuin/goldmark-highlighting@v0.0.0-20200307114337-60d527fdb691/highlighting.go:395 +0x1b7
github.com/yuin/goldmark/renderer.(*renderer).Render.func2(0xc1bf80, 0xc000692000, 0x1, 0x0, 0xc14188, 0xc00059d880)
        ./.go/pkg/mod/github.com/yuin/goldmark@v1.3.3/renderer/renderer.go:166 +0x108
github.com/yuin/goldmark/ast.walkHelper(0xc1bf80, 0xc000692000, 0xc00047fe48, 0x3, 0x0, 0x0)
        ./.go/pkg/mod/github.com/yuin/goldmark@v1.3.3/ast/ast.go:492 +0x43
github.com/yuin/goldmark/ast.walkHelper(0xc1bd40, 0xc0005a18c0, 0xc00047fe48, 0xc000681000, 0x0, 0xc000587600)
        ./.go/pkg/mod/github.com/yuin/goldmark@v1.3.3/ast/ast.go:498 +0x16b
github.com/yuin/goldmark/ast.Walk(...)
        ./.go/pkg/mod/github.com/yuin/goldmark@v1.3.3/ast/ast.go:487
github.com/yuin/goldmark/renderer.(*renderer).Render(0xc00062d0e0, 0xc12068, 0xc000638f30, 0xc0005876d0, 0xe, 0x10, 0xc1bd40, 0xc0005a18c0, 0x10, 0xe)
        ./.go/pkg/mod/github.com/yuin/goldmark@v1.3.3/renderer/renderer.go:161 +0x13c
github.com/yuin/goldmark.(*markdown).Convert(0xc00059d500, 0xc0005876d0, 0xe, 0x10, 0xc12068, 0xc000638f30, 0x0, 0x0, 0x0, 0x406185, ...)
        ./.go/pkg/mod/github.com/yuin/goldmark@v1.3.3/markdown.go:117 +0x12c
main.main()
        ./goldmark_crash.go:21 +0xcb
exit status 2

The parser will crash only when using the goldmark-highlighting extension. But as it seems like the crash is generated within the parser itself, I decided to report the issue in this repo instead.

Extracted a runnable test:

package main

import (
        "bytes"
        "fmt"

        "github.com/yuin/goldmark" // Version v1.3.5
        highlighting "github.com/yuin/goldmark-highlighting" // Version v0.0.0-20210428103930-3a9678dbb86c
)

var gm = goldmark.New(
        goldmark.WithExtensions(
                highlighting.NewHighlighting(),
        ),
)

// This input will crash the parser
var input string = "```0{class=0#."

func main() {
        var buf bytes.Buffer
        err := gm.Convert([]byte(input), &buf)
        fmt.Println("Error:", err)
}

[yuin]

This issue should be reported in the goldmark-higlighting repository.