[Merge to 116] CHECK against overwrites in send_modules_map_

(cherry picked from commit 9d8fb97)

No-try: true
Bug: chromium:1477075
Change-Id: Ia05a868bfab9e99ef66704e8d6bce516a7a43b0a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/318440
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Original-Commit-Position: refs/heads/main@{#40673}
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/319320
Reviewed-by: Philipp Hancke <phancke@microsoft.com>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/branch-heads/5845@{#7}
Cr-Branched-From: f80cf81-refs/heads/main@{#40319}

modules/pacing/packet_router.cc

@@ -68,7 +68,7 @@ void PacketRouter::AddSendRtpModule(RtpRtcpInterface* rtp_module,
 void PacketRouter::AddSendRtpModuleToMap(RtpRtcpInterface* rtp_module,
                                          uint32_t ssrc) {
   RTC_DCHECK_RUN_ON(&thread_checker_);
-  RTC_DCHECK(send_modules_map_.find(ssrc) == send_modules_map_.end());
+  RTC_CHECK(send_modules_map_.find(ssrc) == send_modules_map_.end());
 
   // Signal to module that the pacer thread is attached and can send packets.
   rtp_module->OnPacketSendingThreadSwitched();