Add CHECKs to FlexfecReceiver.

There is a crash happening in this neighbourhood, so adding
CHECKs to tease it out explicitly.

BUG=webrtc:8481

Change-Id: I79a2ec8fd838f4a4735a04496e363b72975919ec
Reviewed-on: https://webrtc-review.googlesource.com/17361
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20520}
diff --git a/modules/rtp_rtcp/source/flexfec_receiver.cc b/modules/rtp_rtcp/source/flexfec_receiver.cc
index ec35a2c..a9a144c 100644
--- a/modules/rtp_rtcp/source/flexfec_receiver.cc
+++ b/modules/rtp_rtcp/source/flexfec_receiver.cc
@@ -10,6 +10,7 @@
 
 #include "modules/rtp_rtcp/include/flexfec_receiver.h"
 
+#include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
 #include "rtc_base/scoped_ref_ptr.h"
 
@@ -129,6 +130,7 @@
 
   // Return recovered packets through callback.
   for (const auto& recovered_packet : recovered_packets_) {
+    RTC_CHECK(recovered_packet);
     if (recovered_packet->returned) {
       continue;
     }
@@ -136,6 +138,7 @@
     // Set this flag first, since OnRecoveredPacket may end up here
     // again, with the same packet.
     recovered_packet->returned = true;
+    RTC_CHECK(recovered_packet->pkt);
     recovered_packet_receiver_->OnRecoveredPacket(
         recovered_packet->pkt->data, recovered_packet->pkt->length);
     // Periodically log the incoming packets.