Fix nil pointer dereference error when open link with invalid pull index (#26353)

fix #26331 

Before:

![image](https://github.com/go-gitea/gitea/assets/50507092/028e6944-84d1-4404-80b6-4a4accdc7d0a)

After:

![image](https://github.com/go-gitea/gitea/assets/50507092/b78978f9-e77f-459f-96e1-3a1f36ec8abe)

routers/web/repo/pull.go

@@ -299,7 +299,7 @@ func ForkPost(ctx *context.Context) {
 	ctx.Redirect(ctxUser.HomeLink() + "/" + url.PathEscape(repo.Name))
 }
 
-func checkPullInfo(ctx *context.Context) *issues_model.Issue {
+func getPullInfo(ctx *context.Context) (issue *issues_model.Issue, ok bool) {
 	issue, err := issues_model.GetIssueByIndex(ctx, ctx.Repo.Repository.ID, ctx.ParamsInt64(":index"))
 	if err != nil {
 		if issues_model.IsErrIssueNotExist(err) {
@@ -307,43 +307,43 @@ func checkPullInfo(ctx *context.Context) *issues_model.Issue {
 		} else {
 			ctx.ServerError("GetIssueByIndex", err)
 		}
-		return nil
+		return nil, false
 	}
 	if err = issue.LoadPoster(ctx); err != nil {
 		ctx.ServerError("LoadPoster", err)
-		return nil
+		return nil, false
 	}
 	if err := issue.LoadRepo(ctx); err != nil {
 		ctx.ServerError("LoadRepo", err)
-		return nil
+		return nil, false
 	}
 	ctx.Data["Title"] = fmt.Sprintf("#%d - %s", issue.Index, issue.Title)
 	ctx.Data["Issue"] = issue
 
 	if !issue.IsPull {
 		ctx.NotFound("ViewPullCommits", nil)
-		return nil
+		return nil, false
 	}
 
 	if err = issue.LoadPullRequest(ctx); err != nil {
 		ctx.ServerError("LoadPullRequest", err)
-		return nil
+		return nil, false
 	}
 
 	if err = issue.PullRequest.LoadHeadRepo(ctx); err != nil {
 		ctx.ServerError("LoadHeadRepo", err)
-		return nil
+		return nil, false
 	}
 
 	if ctx.IsSigned {
 		// Update issue-user.
 		if err = activities_model.SetIssueReadBy(ctx, issue.ID, ctx.Doer.ID); err != nil {
 			ctx.ServerError("ReadBy", err)
-			return nil
+			return nil, false
 		}
 	}
 
-	return issue
+	return issue, true
 }
 
 func setMergeTarget(ctx *context.Context, pull *issues_model.PullRequest) {
@@ -361,14 +361,15 @@ func setMergeTarget(ctx *context.Context, pull *issues_model.PullRequest) {
 
 // GetPullDiffStats get Pull Requests diff stats
 func GetPullDiffStats(ctx *context.Context) {
-	issue := checkPullInfo(ctx)
+	issue, ok := getPullInfo(ctx)
+	if !ok {
+		return
+	}
 	pull := issue.PullRequest
 
 	mergeBaseCommitID := GetMergedBaseCommitID(ctx, issue)
 
-	if ctx.Written() {
+	if mergeBaseCommitID == "" {
-		return
-	} else if mergeBaseCommitID == "" {
 		ctx.NotFound("PullFiles", nil)
 		return
 	}
@@ -702,8 +703,8 @@ type pullCommitList struct {
 
 // GetPullCommits get all commits for given pull request
 func GetPullCommits(ctx *context.Context) {
-	issue := checkPullInfo(ctx)
+	issue, ok := getPullInfo(ctx)
-	if ctx.Written() {
+	if !ok {
 		return
 	}
 	resp := &pullCommitList{}
@@ -735,8 +736,8 @@ func ViewPullCommits(ctx *context.Context) {
 	ctx.Data["PageIsPullList"] = true
 	ctx.Data["PageIsPullCommits"] = true
 
-	issue := checkPullInfo(ctx)
+	issue, ok := getPullInfo(ctx)
-	if ctx.Written() {
+	if !ok {
 		return
 	}
 	pull := issue.PullRequest
@@ -779,8 +780,8 @@ func viewPullFiles(ctx *context.Context, specifiedStartCommit, specifiedEndCommi
 	ctx.Data["PageIsPullList"] = true
 	ctx.Data["PageIsPullFiles"] = true
 
-	issue := checkPullInfo(ctx)
+	issue, ok := getPullInfo(ctx)
-	if ctx.Written() {
+	if !ok {
 		return
 	}
 	pull := issue.PullRequest
@@ -1016,8 +1017,8 @@ func ViewPullFilesForAllCommitsOfPr(ctx *context.Context) {
 
 // UpdatePullRequest merge PR's baseBranch into headBranch
 func UpdatePullRequest(ctx *context.Context) {
-	issue := checkPullInfo(ctx)
+	issue, ok := getPullInfo(ctx)
-	if ctx.Written() {
+	if !ok {
 		return
 	}
 	if issue.IsClosed {
@@ -1101,8 +1102,8 @@ func UpdatePullRequest(ctx *context.Context) {
 // MergePullRequest response for merging pull request
 func MergePullRequest(ctx *context.Context) {
 	form := web.GetForm(ctx).(*forms.MergePullRequestForm)
-	issue := checkPullInfo(ctx)
+	issue, ok := getPullInfo(ctx)
-	if ctx.Written() {
+	if !ok {
 		return
 	}
 
@@ -1308,8 +1309,8 @@ func MergePullRequest(ctx *context.Context) {
 
 // CancelAutoMergePullRequest cancels a scheduled pr
 func CancelAutoMergePullRequest(ctx *context.Context) {
-	issue := checkPullInfo(ctx)
+	issue, ok := getPullInfo(ctx)
-	if ctx.Written() {
+	if !ok {
 		return
 	}
 
@@ -1447,8 +1448,8 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 
 // CleanUpPullRequest responses for delete merged branch when PR has been merged
 func CleanUpPullRequest(ctx *context.Context) {
-	issue := checkPullInfo(ctx)
+	issue, ok := getPullInfo(ctx)
-	if ctx.Written() {
+	if !ok {
 		return
 	}
 

routers/web/repo/pull_review.go

@@ -259,8 +259,8 @@ type viewedFilesUpdate struct {
 
 func UpdateViewedFiles(ctx *context.Context) {
 	// Find corresponding PR
-	issue := checkPullInfo(ctx)
+	issue, ok := getPullInfo(ctx)
-	if ctx.Written() {
+	if !ok {
 		return
 	}
 	pull := issue.PullRequest