markup/goldmark/codeblocks: Fix slice bounds out of range

For the Position in code blocks we try to match the .Inner with the original source. This isn't always possible.

This commits avoids panics in these situations.

Fixes #9627

hugolib/page.go

@@ -784,6 +784,11 @@ func (p *pageState) posFromPage(offset int) text.Position {
 }
 
 func (p *pageState) posFromInput(input []byte, offset int) text.Position {
+	if offset < 0 {
+		return text.Position{
+			Filename: p.pathOrTitle(),
+		}
+	}
 	lf := []byte("\n")
 	input = input[:offset]
 	lineNumber := bytes.Count(input, lf) + 1

markup/goldmark/codeblocks/integration_test.go

@@ -164,6 +164,43 @@ fmt.Println("Hello, World!");
 	)
 }
 
+func TestCodeblocksBugs(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- config.toml --
+-- layouts/_default/_markup/render-codeblock.html --
+{{ .Position | safeHTML }}
+-- layouts/_default/single.html --
+{{ .Content }}
+-- content/p1.md --
+---
+title: "p1"
+---
+
+## Issue 9627
+
+§§§text
+{{</* foo */>}}
+§§§
+
+`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+			NeedsOsFS:   false,
+		},
+	).Build()
+
+	b.AssertFileContent("public/p1/index.html", `
+# Issue 9627: For the Position in code blocks we try to match the .Inner with the original source. This isn't always possible.
+p1.md:0:0
+	`,
+	)
+}
+
 func TestCodeChomp(t *testing.T) {
 	t.Parallel()