Fix a bug in RtpFileSource related to RTCP packets in rtpdump files

According to http://www.cs.columbia.edu/irt/software/rtptools/#rtpdump,
RTCP packets are marked with plen==0. In this class, plen is mapped to
original_length, not length.

Review URL: https://codereview.webrtc.org/1356543002

Cr-Commit-Position: refs/heads/master@{#9981}

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

@@ -47,7 +47,7 @@ Packet* RtpFileSource::NextPacket() {
     if (!rtp_reader_->NextPacket(&temp_packet)) {
       return NULL;
     }
-    if (temp_packet.length == 0) {
+    if (temp_packet.original_length == 0) {
       // May be an RTCP packet.
       // Read the next one.
       continue;