Use memoizedPagechecker (#13191)

It seems like this was supposed to use the memoized version of `pageChecker`. I also updated the function to memoize promises instead of promise results. Just in case this function ever gets called concurrently.

packages/next/next-server/server/router.ts

@@ -162,12 +162,12 @@ export default class Router {
     parsedUrl: UrlWithParsedQuery
   ): Promise<boolean> {
     // memoize page check calls so we don't duplicate checks for pages
-    const pageChecks: { [name: string]: boolean } = {}
+    const pageChecks: { [name: string]: Promise<boolean> } = {}
     const memoizedPageChecker = async (p: string): Promise<boolean> => {
       if (pageChecks[p]) {
         return pageChecks[p]
       }
-      const result = await this.pageChecker(p)
+      const result = this.pageChecker(p)
       pageChecks[p] = result
       return result
     }
@@ -200,7 +200,7 @@ export default class Router {
                 if (!pathname) {
                   return { finished: false }
                 }
-                if (await this.pageChecker(pathname)) {
+                if (await memoizedPageChecker(pathname)) {
                   return this.catchAllRoute.fn(req, res, params, parsedUrl)
                 }
                 return { finished: false }