neteq_rtpplay: Add an error message for unmatched SSRC

If neteq_rtpplay is invoked with the --ssrc option to select packets
matching a specific SSRC, but no matching packets are found, this CL
provides a meaningful error message.

BUG=webrtc:2692
NOTRY=True
TBR=ivoc@webrtc.org

Review-Url: https://codereview.webrtc.org/2318503002
Cr-Commit-Position: refs/heads/master@{#14083}

webrtc/modules/audio_coding/neteq/tools/neteq_rtpplay.cc

@@ -15,6 +15,7 @@
 #include <stdlib.h>  // For strtoul.
 
 #include <algorithm>
+#include <ios>
 #include <iostream>
 #include <memory>
 #include <string>
@@ -245,6 +246,8 @@ class FilterSsrcInput : public NetEqInput {
   FilterSsrcInput(std::unique_ptr<NetEqInput> source, uint32_t ssrc)
       : source_(std::move(source)), ssrc_(ssrc) {
     FindNextWithCorrectSsrc();
+    RTC_CHECK(source_->NextHeader()) << "Found no packet with SSRC = 0x"
+                                     << std::hex << ssrc_;
   }
 
   // All methods but PopPacket() simply relay to the |source_| object.