network_error_code isn't parsed

[c-bik]

fixes https://github.com/K2InformaticsGmbH/mpro/issues/163

> S =
"00 00 00 C3 00 00 00 05 00 00 00 00 00 00 00 01 00 01 01 31 31 31 31 31 31 31 31 31 31 31 00 00 "
"01 31 31 31 31 00 04 00 00 00 00 00 00 00 00 7A 69 64 3A 31 34 38 39 37 32 34 32 31 32 20 73 75 "
"62 3A 30 30 31 20 64 6C 76 72 64 3A 30 30 30 20 73 75 62 6D 69 74 20 64 61 74 65 3A 31 37 31 30 "
"30 32 31 33 34 33 20 64 6F 6E 65 20 64 61 74 65 3A 31 37 31 30 30 32 31 33 34 34 20 73 74 61 74 "
"3A 45 58 50 49 52 45 44 20 65 72 72 3A 30 30 36 20 74 65 78 74 3A 48 65 6C 6C 6F 20 57 6F 72 6C "
"64 2C 20 49 27 6D 20 61 20 73 00 1E 00 09 35 38 43 42 36 33 33 34 00 04 23 00 03 03 00 06 04 27 "
"00 01 03".

10> smpp:unpack(S).    
Rec info not defined for type : network_error_code
[{command_id,5},
 {command_status,0},
 {sequence_number,1},
 {message_state,3},
 {network_error_code,[]},
 {receipted_message_id,"58CB6334"},
 {service_type,[]},
 {source_addr_ton,1},
 {source_addr_npi,1},
 {source_addr,"11111111111"},
 {dest_addr_ton,0},
 {dest_addr_npi,1},
 {destination_addr,"1111"},
 {esm_class,4},
 {protocol_id,0},
 {priority_flag,0},
 {schedule_delivery_time,[]},
 {validity_period,[]},
 {registered_delivery,0},
 {replace_if_present_flag,0},
 {data_coding,0},
 {sm_default_msg_id,0},
 {short_message,"id:1489724212 sub:001 dlvrd:000 submit date:1710021343 done date:1710021344 stat:EXPIRED err:006 text:Hello World, I'm a s"}]

11> smpp:unpack_map(S).
Rec info not defined for type : network_error_code
#{command_id => 5,command_status => 0,data_coding => 0,
  dest_addr_npi => 1,dest_addr_ton => 0,
  destination_addr => "1111",esm_class => 4,
  message_state => 3,network_error_code => #{},
  priority_flag => 0,protocol_id => 0,
  receipted_message_id => "58CB6334",registered_delivery => 0,
  replace_if_present_flag => 0,schedule_delivery_time => [],
  sequence_number => 1,service_type => [],
  short_message =>
      "id:1489724212 sub:001 dlvrd:000 submit date:1710021343 done date:1710021344 stat:EXPIRED err:006 text:Hello World, I'm a s",
  sm_default_msg_id => 0,source_addr => "11111111111",
  source_addr_npi => 1,source_addr_ton => 1,
  validity_period => []}