Fix infinite loop in rindexes with maxless data

Verify that index is not undefined in isValidIndex()

Fixes a bug introduced in c29079d, when support for non-preallocated
data bound by ttl and/or maxSize was added.  When the pointer array is a
`number[]` instead of a `UintArray`, unset values will be `undefined`
rather than `0`, resulting in the situation where `index ===
this.keyMap.get(this.keyList[index])` is not a valid test for index
validity, since it can be `undefined` rather than always being
guaranteed to be a number.

Another way to fix this would be ensure that a previously unused spot in
the array is initialized to `0` when pointers are initialized.  However,
in performance critical cases, every set and get matters, and relying on
the initialized state of a UintArray saves that check in several hot
paths.  On the other hand, the test for `index === undefined` only has
to happen during iteration, which is much less likely to be performance
critical.

Fix: #278

index.js

@@ -488,7 +488,10 @@ class LRUCache {
   }
 
   isValidIndex(index) {
-    return this.keyMap.get(this.keyList[index]) === index
+    return (
+      index !== undefined &&
+      this.keyMap.get(this.keyList[index]) === index
+    )
   }
 
   *entries() {

test/reverse-iterate-delete-all.ts

@@ -0,0 +1,13 @@
+// https://github.com/isaacs/node-lru-cache/issues/278
+import t from 'tap'
+import LRU from '../'
+const lru = new LRU<string, string>({
+  maxSize:2,
+  sizeCalculation: () => 1,
+});
+lru.set('x', 'x')
+lru.set('y', 'y')
+for (const key of lru.rkeys()) {
+  lru.delete(key)
+}
+t.equal(lru.size, 0)