Fix a panic bug when head repository deleting (#30674)

When visiting a pull request files which head repository has been
deleted, it will panic because headrepo is nil.

routers/web/repo/pull.go

@@ -863,21 +863,21 @@ func viewPullFiles(ctx *context.Context, specifiedStartCommit, specifiedEndCommi
 
 		if pull.HeadRepo != nil {
 			ctx.Data["SourcePath"] = pull.HeadRepo.Link() + "/src/branch/" + util.PathEscapeSegments(pull.HeadBranch)
-		}
 
-		if !pull.HasMerged && ctx.Doer != nil {
-			perm, err := access_model.GetUserRepoPermission(ctx, pull.HeadRepo, ctx.Doer)
-			if err != nil {
-				ctx.ServerError("GetUserRepoPermission", err)
-				return
-			}
+			if !pull.HasMerged && ctx.Doer != nil {
+				perm, err := access_model.GetUserRepoPermission(ctx, pull.HeadRepo, ctx.Doer)
+				if err != nil {
+					ctx.ServerError("GetUserRepoPermission", err)
+					return
+				}
 
-			if perm.CanWrite(unit.TypeCode) || issues_model.CanMaintainerWriteToBranch(ctx, perm, pull.HeadBranch, ctx.Doer) {
-				ctx.Data["CanEditFile"] = true
-				ctx.Data["EditFileTooltip"] = ctx.Tr("repo.editor.edit_this_file")
-				ctx.Data["HeadRepoLink"] = pull.HeadRepo.Link()
-				ctx.Data["HeadBranchName"] = pull.HeadBranch
-				ctx.Data["BackToLink"] = setting.AppSubURL + ctx.Req.URL.RequestURI()
+				if perm.CanWrite(unit.TypeCode) || issues_model.CanMaintainerWriteToBranch(ctx, perm, pull.HeadBranch, ctx.Doer) {
+					ctx.Data["CanEditFile"] = true
+					ctx.Data["EditFileTooltip"] = ctx.Tr("repo.editor.edit_this_file")
+					ctx.Data["HeadRepoLink"] = pull.HeadRepo.Link()
+					ctx.Data["HeadBranchName"] = pull.HeadBranch
+					ctx.Data["BackToLink"] = setting.AppSubURL + ctx.Req.URL.RequestURI()
+				}
 			}
 		}
 	}