Fix handling RTCP when there is no track (#170)

Author: roznawsk

lib/ex_webrtc/peer_connection.ex

@@ -2063,6 +2063,9 @@ defmodule ExWebRTC.PeerConnection do
         nil ->
           {nil, state}
 
+        {%{sender: %{track: nil}}, _idx} ->
+          {nil, state}
+
         # in case NACK was received, but RTX was not negotiated
         # as NACK and RTX are negotiated independently
         {%{sender: %{rtx_pt: nil}} = tr, _idx} ->
@@ -2087,6 +2090,9 @@ defmodule ExWebRTC.PeerConnection do
       nil ->
         {nil, state}
 
+      {%{sender: %{track: nil}}, _idx} ->
+        {nil, state}
+
       {tr, idx} ->
         tr = RTPTransceiver.receive_pli(tr, pli)
         transceivers = List.replace_at(state.transceivers, idx, tr)