NPE on show pull with unclear cause

[zeripath]

• Gitea version (or commit ref): 1.13.6
• Git version: ?
• Operating system: ?
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No - https://git.pyrocko.org/pyrocko/pyrocko/pulls/159
• Log gist:

template: repo/issue/view_content/comments:470:20: executing "repo/issue/view_content/comments" at <CommentMustAsDiff (index $comms 0)>: error calling CommentMustAsDiff: PANIC whilst retrieving diff for comment[775] Error: runtime error: invalid memory address or nil pointer dereference
Stack: /usr/local/go/src/runtime/panic.go:212 (0x4565b2)
/usr/local/go/src/runtime/signal_unix.go:742 (0x456432)
/go/src/code.gitea.io/gitea/services/gitdiff/gitdiff.go:1029 (0x1830a99)
/go/src/code.gitea.io/gitea/services/gitdiff/gitdiff.go:856 (0x182eb04)
/go/src/code.gitea.io/gitea/services/gitdiff/gitdiff.go:1267 (0x183357c)
/go/src/code.gitea.io/gitea/services/gitdiff/gitdiff.go:1295 (0x1833867)
/usr/local/go/src/reflect/value.go:476 (0x4a6fe6)

Description

There is an issue whereby some code review comments can cause a panic when trying to render the comments on the page.

These issues were reported by @miili @skillcoder on #14812 but I have not been able to get more information from them regarding this. (In particular it would be helpful to see examples of the comment table to figure this out.) I have even emailed the administrator of the pyrocko server and have had no response.

However, #15086 has allowed me to discover what the proximal issue is despite the lack of further information. It appears that there is an NPE here:

gitea/services/gitdiff/gitdiff.go

Line 1029 in 8d4f8eb

curSection.Lines = append(curSection.Lines, diffLine)

I don't understand how this can happen as there should always be a hunk header line - however, without actually seeing the patch that causes the problem or any way to replicate it on master/1.14/1.13 I have little other option than to dumbly attempt to work around the issue.

So please @miili @skillcoder would you show me the contents of the comment table that are associated with the issue that cause this.