Fix "control reaches end of non-void function" warnings

"warning: control reaches end of non-void function [-Wreturn-type]"
Reported by gcc (8.3)

In all the reported cases, the end of function is never actually
reached. Add RTC_CHECK(false) to ensure the compiler is aware that
this path is a dead-end.

Bug: webrtc:12008
Change-Id: I7f816fde3d1897ed2774057c7e05da66e1895e60
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/189784
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Fabien VALLÉE <fabien.vallee@netgem.com>
Cr-Commit-Position: refs/heads/master@{#32503}

modules/rtp_rtcp/source/create_video_rtp_depacketizer.cc

@@ -37,6 +37,7 @@ std::unique_ptr<VideoRtpDepacketizer> CreateVideoRtpDepacketizer(
     case kVideoCodecMultiplex:
       return std::make_unique<VideoRtpDepacketizerGeneric>();
   }
+  RTC_CHECK(false);
 }
 
 }  // namespace webrtc