stream: add bounds checking to RingBuffer.get()

Return `undefined` for out-of-bounds indices instead of wrapping
into stale backing array slots via modular arithmetic.

Author: jasnell

lib/internal/streams/iter/ringbuffer.js

@@ -65,9 +65,11 @@ class RingBuffer {
 
   /**
    * Read item at a logical index (0 = head). O(1).
+   * Returns undefined if index is out of bounds.
    * @returns {any}
    */
   get(index) {
+    if (index < 0 || index >= this.#size) return undefined;
     return this.#backing[(this.#head + index) % this.#capacity];
   }