Fix panic parsing list definition

[jfontan]

This file causes panic on parse:

https://raw.githubusercontent.com/carnot-technologies/redis-scaling/master/README.md

The panic:

panic: runtime error: index out of range [709] with length 709 [recovered]
    panic: runtime error: index out of range [709] with length 709

goroutine 18 [running]:
testing.tRunner.func1.2({0x52c000, 0xc0001fc000})
    /usr/lib/go/src/testing/testing.go:1209 +0x24e
testing.tRunner.func1()
    /usr/lib/go/src/testing/testing.go:1212 +0x218
panic({0x52c000, 0xc0001fc000})
    /usr/lib/go/src/runtime/panic.go:1038 +0x215
github.com/russross/blackfriday/v2.(*Markdown).listItem(0xc0001d0900, {0xc0001c3daa, 0x2c5, 0x0}, 0xc00009acd0)
    /home/jfontan/git/blackfriday/block.go:1372 +0xd9b
github.com/russross/blackfriday/v2.(*Markdown).list(0xc0001d0900, {0xc0001c3daa, 0x2c5, 0x2c6}, 0x16)
    /home/jfontan/git/blackfriday/block.go:1135 +0xdd
github.com/russross/blackfriday/v2.(*Markdown).paragraph(0xc0001d0900, {0xc0001c3d6e, 0x301, 0x302})
    /home/jfontan/git/blackfriday/block.go:1485 +0x525
github.com/russross/blackfriday/v2.(*Markdown).block(0xc0001d0900, {0xc0001c2000, 0x40cf47, 0x10})
    /home/jfontan/git/blackfriday/block.go:194 +0x671
github.com/russross/blackfriday/v2.(*Markdown).Parse(0xc0001d0900, {0xc0001c2000, 0x0, 0x0})
    /home/jfontan/git/blackfriday/markdown.go:404 +0x27
github.com/russross/blackfriday/v2.Run({0xc0001c2000, 0x206f, 0x2070}, {0x0, 0x0, 0x465cf3})
    /home/jfontan/git/blackfriday/markdown.go:388 +0x1f9
github.com/russross/blackfriday/v2.TestBadReadme(0xc000083380)
    /home/jfontan/git/blackfriday/badreadme_test.go:14 +0x98
testing.tRunner(0xc000083380, 0x5553d0)
    /usr/lib/go/src/testing/testing.go:1259 +0x102
created by testing.(*T).Run
    /usr/lib/go/src/testing/testing.go:1306 +0x35a

I've changed the code so the variable next is never larger or equal than data size. I'm not sure is the correct fix as it treats previous text to a definition list as a dl. At least it does not crash now.