Handle empty client_handler

[olegokunevych]

We are using membrane_rtmp_plugin lib as part of our pipeline. Occasionally, once or twice a day plugin crashes with the following error:

UndefinedFunctionError: function nil.handle_connection_closed/1 is undefined
  ?, in nil.handle_connection_closed/1
  File "lib/membrane_rtmp_plugin/rtmp_server/client_handler.ex", line 238, in Membrane.RTMPServer.ClientHandler.handle_event/2
  File "lib/enum.ex", line 2546, in Enum."-reduce/3-lists^foldl/2-0-"/3
  File "lib/membrane_rtmp_plugin/rtmp_server/client_handler.ex", line 118, in Membrane.RTMPServer.ClientHandler.handle_info/2
  File "gen_server.erl", line 1095, in :gen_server.try_handle_info/3
  File "gen_server.erl", line 1183, in :gen_server.handle_msg/6
  File "proc_lib.erl", line 241, in :proc_lib.init_p_do_apply/3

I've been able to reproduce it locally, and this error happens even without any RTMP connections. @varsill could you take a closer look and find out if this fix makes sense?

[varsill]

Hi @olegokunevych !
Thank you very much for the contribution!
It turns out that the socket was getting closed before the handler had been created (creating handler requires parsing some RTMP messages).