Reset indexer upon initialization in AudioLoop.

The array is reset in Init() but not the indexer. This makes the start point undefined after Init() for re-initializing an AudioLoop. This can be fixed.

BUG=

Review URL: https://codereview.webrtc.org/1727353002

Cr-Commit-Position: refs/heads/master@{#11739}

webrtc/modules/audio_coding/neteq/tools/audio_loop.cc

@@ -40,6 +40,7 @@ bool AudioLoop::Init(const std::string file_name,
 
   loop_length_samples_ = samples_read;
   block_length_samples_ = block_length_samples;
+  next_index_ = 0;
   return true;
 }