fix(HLS): Fix decryption of AES-128 (#5871)

Related to
#5785 (comment)

lib/media/streaming_engine.js

@@ -1523,11 +1523,11 @@ shaka.media.StreamingEngine = class {
   /**
    * @param {!BufferSource} rawResult
    * @param {shaka.extern.aes128Key} aes128Key
-   * @param {!shaka.media.SegmentIterator} iter
+   * @param {number} position
    * @return {!Promise.<!BufferSource>} finalResult
    * @private
    */
-  async aes128Decrypt_(rawResult, aes128Key, iter) {
+  async aes128Decrypt_(rawResult, aes128Key, position) {
     const key = aes128Key;
     if (!key.cryptoKey) {
       goog.asserts.assert(key.fetchKey, 'If AES-128 cryptoKey was not ' +
@@ -1538,7 +1538,7 @@ shaka.media.StreamingEngine = class {
     let iv = key.iv;
     if (!iv) {
       iv = shaka.util.BufferUtils.toUint8(new ArrayBuffer(16));
-      let sequence = key.firstMediaSequenceNumber + iter.currentPosition();
+      let sequence = key.firstMediaSequenceNumber + position;
       for (let i = iv.byteLength - 1; i >= 0; i--) {
         iv[i] = sequence & 0xff;
         sequence >>= 8;
@@ -2214,18 +2214,17 @@ shaka.media.StreamingEngine = class {
           reference, mediaState.stream, streamDataCallback);
     }
 
+    let position = 0;
+    if (mediaState.segmentIterator) {
+      position = mediaState.segmentIterator.currentPosition();
+    }
+
     mediaState.operation = op;
     const response = await op.promise;
     mediaState.operation = null;
     let result = response.data;
     if (reference.aes128Key) {
-      let iter = mediaState.segmentIterator;
-      const time = reference.startTime;
-      if (!iter && time != null) {
-        iter = mediaState.stream.segmentIndex.getIteratorForTime(time);
-      }
-      goog.asserts.assert(iter, 'mediaState.segmentIterator should exist');
-      result = await this.aes128Decrypt_(result, reference.aes128Key, iter);
+      result = await this.aes128Decrypt_(result, reference.aes128Key, position);
     }
     return result;
   }