Fix for 3 NetEq fuzzer issues.

I was not able to reproduce chromium:1146676 locally, so the change in merge.cc is a speculative fix.

Bug: chromium:1146835, chromium:1146676, chromium:1137226
Change-Id: I14472ba5b41e58b2d5f27d9833249c14505af18f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/194264
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32759}
diff --git a/modules/audio_coding/neteq/merge.cc b/modules/audio_coding/neteq/merge.cc
index f1f2cc9..5bf239b 100644
--- a/modules/audio_coding/neteq/merge.cc
+++ b/modules/audio_coding/neteq/merge.cc
@@ -50,6 +50,9 @@
   assert(fs_hz_ == 8000 || fs_hz_ == 16000 || fs_hz_ == 32000 ||
          fs_hz_ == 48000);
   assert(fs_hz_ <= kMaxSampleRate);  // Should not be possible.
+  if (input_length == 0) {
+    return 0;
+  }
 
   size_t old_length;
   size_t expand_period;