Ensure that ctx.Written is checked after issues(...) calls (#15797) (#15798)

Fix issue noted in #15783

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: zeripath <art27@cantab.net>

routers/repo/issue.go

@@ -374,6 +374,9 @@ func Issues(ctx *context.Context) {
 	}
 
 	issues(ctx, ctx.QueryInt64("milestone"), ctx.QueryInt64("project"), util.OptionalBoolOf(isPullList))
+	if ctx.Written() {
+		return
+	}
 
 	var err error
 	// Get milestones