Fix panic in blame view when a file has only a single commit

[0xMax42]

This PR fixes a panic in the repository blame view that occurs when rendering files
whose blame history consists of only a single commit.

In this case, the number of rendered blame rows can exceed the number of
highlighted lines returned by the syntax highlighter. The existing bounds check
incorrectly used len(rows), which may be greater than len(unsafeLines),
leading to an out-of-range access on unsafeLines and a runtime panic.

The fix changes the bounds check to use len(unsafeLines) instead, ensuring we
never index past the available highlighted lines.

This issue is reproducible with files that:

• were introduced in exactly one commit, and
• are accessed via the blame view (e.g. /blame/commit/<sha>/file).

The change is minimal, safe, and does not affect normal blame rendering.

---

Example error:

2025/12/22 02:35:31 HTTPRequest [W] router: failed    GET /actions/deb-changelog-action/blame/commit/1239d674128fe38b284802f4e97c70068116fd52/CHANGELOG.md for XXX.XXX.XXX.XXX:0, panic in 34.8ms @ repo/blame.go:44(repo.RefBlame), err=runtime error: index out of range [61] with length 61
2025/12/22 02:35:31 routers/common/errpage.go:25:RenderPanicErrorPage() [E] PANIC: runtime error: index out of range [61] with length 61
/usr/local/go/src/runtime/panic.go:783 (0x4860b1)
/go/src/code.gitea.io/gitea/modules/web/routing/logger_manager.go:116 (0x19b8a84)
/usr/local/go/src/runtime/panic.go:783 (0x4860b1)
/usr/local/go/src/runtime/panic.go:115 (0x44a1b3)
/go/src/code.gitea.io/gitea/routers/web/repo/blame.go:275 (0x2aaa01a)
/go/src/code.gitea.io/gitea/routers/web/repo/blame.go:97 (0x2aa8a08)
[...]

Introduced with PR #36157