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

@@ -863,7 +863,6 @@ 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)
@@ -881,6 +880,7 @@ func viewPullFiles(ctx *context.Context, specifiedStartCommit, specifiedEndCommi
 				}
 			}
 		}
+	}
 
 	ctx.HTML(http.StatusOK, tplPullFiles)
 }