NetEq: Handle nested RED packets

This CL makes NetEq handle nested RED packets without crashing. Nested
RED packets mean that the block PT (see
https://tools.ietf.org/html/rfc2198.html#section-3) in the RED packet
is also set to the RED PT. This implies a nested RED packet, which is
not supported. Instead, all payloads in a RED packet that have the RED
PT will be discarded.

Bug: chromium:851662
Change-Id: I86ec257e60fb8076e3574ac5a4a1ca50196f6b34
Reviewed-on: https://webrtc-review.googlesource.com/86824
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23825}

modules/audio_coding/neteq/neteq_impl.cc

@@ -612,6 +612,9 @@ int NetEqImpl::InsertPacketInternal(const RTPHeader& rtp_header,
     // Only accept a few RED payloads of the same type as the main data,
     // DTMF events and CNG.
     red_payload_splitter_->CheckRedPayloads(&packet_list, *decoder_database_);
+    if (packet_list.empty()) {
+      return kRedundancySplitError;
+    }
   }
 
   // Check payload types.