Zero extra bytes of FEC recovered packet

rtc::CopyOnWriteBuffer::SetSize extends buffer with uninitialized memory by design.
It is up to the user of the rtc::CopyOnWriteBuffer to ensure it is initialized.

(cherry picked from commit f52e015)

No-Try: true
Bug: chromium:1403397
Change-Id: Ic0111a84bda32379770ddb1c7d24bee10d96b7a4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/289041
Reviewed-by: Rasmus Brandt <[email protected]>
Commit-Queue: Danil Chapovalov <[email protected]>
Cr-Original-Commit-Position: refs/heads/main@{#38959}
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/291540
Reviewed-by: Per Kjellander <[email protected]>
Cr-Commit-Position: refs/branch-heads/5481@{#3}
Cr-Branched-From: 2e1a9a4-refs/heads/main@{#38901}

Author: DanilChapovalov

modules/rtp_rtcp/source/forward_error_correction.cc

@@ -573,7 +573,13 @@ bool ForwardErrorCorrection::FinishPacketRecovery(
                            "typical IP packet, and is thus dropped.";
     return false;
   }
+  size_t old_size = recovered_packet->pkt->data.size();
   recovered_packet->pkt->data.SetSize(new_size);
+  data = recovered_packet->pkt->data.MutableData();
+  if (new_size > old_size) {
+    memset(data + old_size, 0, new_size - old_size);
+  }
+
   // Set the SN field.
   ByteWriter<uint16_t>::WriteBigEndian(&data[2], recovered_packet->seq_num);
   // Set the SSRC field.