Fix incorrect comment diff hunk parsing, fix github asset ID nil panic (#35046)

* Fix missing the first char when parsing diff hunk header
* Fix #35040
* Fix #35049

----

Introduced in
https://github.com/go-gitea/gitea/pull/12047/files#diff-de48c2f70e24ff5603180acf8b5ce9d0356ede8a45bfbf2a485707282ace6d6aR268

Before:

<img width="487" height="167" alt="image"
src="https://github.com/user-attachments/assets/17524c76-a296-4b4b-a4f9-c5150c41bae5"
/>

After:

<img width="749" height="144" alt="image"
src="https://github.com/user-attachments/assets/bcb12c76-c1ae-40f1-81b7-183d15f891db"
/>

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/migrations/github.go

@@ -322,7 +322,10 @@ func (g *GithubDownloaderV3) convertGithubRelease(ctx context.Context, rel *gith
 	httpClient := NewMigrationHTTPClient()
 
 	for _, asset := range rel.Assets {
-		assetID := *asset.ID // Don't optimize this, for closure we need a local variable
+		assetID := asset.GetID() // Don't optimize this, for closure we need a local variable TODO: no need to do so in new Golang
+		if assetID == 0 {
+			continue
+		}
 		r.Assets = append(r.Assets, &base.ReleaseAsset{
 			ID:            asset.GetID(),
 			Name:          asset.GetName(),