Check ctx.Written() for GetActionIssue (#25698) (#25711)

Backport #25698 by @wolfogre

Fix #25697.

Just avoid panic, maybe there's another bug to trigger this case.

Co-authored-by: Jason Song <i@wolfogre.com>

routers/web/repo/pull.go

@@ -1470,10 +1470,10 @@ func DownloadPullDiffOrPatch(ctx *context.Context, patch bool) {
 // UpdatePullRequestTarget change pull request's target branch
 func UpdatePullRequestTarget(ctx *context.Context) {
 	issue := GetActionIssue(ctx)
-	pr := issue.PullRequest
 	if ctx.Written() {
 		return
 	}
+	pr := issue.PullRequest
 	if !issue.IsPull {
 		ctx.Error(http.StatusNotFound)
 		return