Resolved: Incorrect connection count handling

pianod crash logs showing connection from 0.0.0.0
In the last seconds before the 2021-01-30 crash
2021-01-30 07:22:20: fb_service.c:337 (fb_validate_connection): #16: Connection from 127.0.0.1 permitted.
2021-01-30 07:22:20: fb_event.c:530 (fb_new_connect): #16: New cleartext IP4 line connection from 127.0.0.1 port 60330
2021-01-30 07:22:20: fb_service.c:536 (fb_destroy_connection): #16: Connection terminated.
2021-01-30 07:22:20: engine.cpp:442 PurgeUnselectedSongs (LOG_GENERAL): Removing song 3ss390771106-11 ('Erstes Dreigroschen-Finale') from random queue: song's playlist is not in the mix
2021-01-30 07:22:20: engine.cpp:442 PurgeUnselectedSongs (LOG_GENERAL): Removing song 3ss537272334-1 ('Tchaikovsky -  Concerto in D, Op. 35 - Allegro moderato - Heifetz') from random queue: song's playlist is not in the mix
2021-01-30 07:22:20: engine.cpp:442 PurgeUnselectedSongs


Crash #2 on 2021-02-02, last seconds:
2021-02-02 05:49:32: fb_http.c:1108 (http_serve_data): #20: pianod: HTTP request: GET translate.js (200 Ok)
2021-02-02 05:49:32: fb_http.c:1108 (http_serve_data): #21: pianod: HTTP request: GET client.lib.js (200 Ok)
2021-02-02 05:49:32: fb_http.c:1108 (http_serve_data): #20: pianod: HTTP request: GET no-art.jpeg (200 Ok)
2021-02-02 05:49:32: fb_service.c:337 (fb_validate_connection): #22: Connection from 10.69.69.254 permitted.
2021-02-02 05:49:32: fb_event.c:529 (fb_new_connect): #22: New cleartext IP4 HTTP connection from 10.69.69.254 port 37852
2021-02-02 05:49:32: fb_http.c:775 (fb_greet_websocket): #22: pianod WebSocket session initiated
2021-02-02 05:49:32: fb_http.c:1108 (http_serve_data): #21: pianod: HTTP request: GET pianod-button.gif (200 Ok)
2021-02-02 05:49:32: fb_http.c:1108 (http_serve_data): #20: pianod: HTTP request: GET pianod-icon.gif (200 Ok)
2021-02-02 05:49:32: fb_service.c:536 (fb_destroy_connection): #20: Connection terminated.

Similarities:
- Both crashes are preceeded by fb_destroy_connection
- Both crashes are in the same function, same line
Differences: 
- The 2021-01-30 crash, fb_destroy_connection is closure of a line connection to pianod.
- The 2021-01-30 crash occurs from periodic(), not from the command dispatcher.
- The 2021-02-02 crash is closing an HTTP connection that is connected to Football only.
- The 2021-02-02 crash occurs during a command dispatch.


Crash happening:
bool User::online (const PianodService &service) const {
    std::vector <PianodConnection *> connections;
    for (auto conn : service) {
        if (conn->user == this) { ///< CRASH HAPPENING HERE
            return true;
        }
    }
    return false;
}


This is a pointer comparison, suggesting `conn` is either NULL or
invalid.

Could the iterator have returned something invalid?
- Iterator only yields members with a non-null related object.
- Related object is destroyed and properly nulled out in fb_arena.cpp
  case handler.
- Related object is parts of FB_CONNECTION structure, which is `calloc`ed and
  thus should be NULL to start.
- Football's C++ connection object gets destroyed.
- So what removes the connection from the service?
- The iterator is looking at the C FB_SERVICE structure.
- relatedObject is set in getFromOld, built by service's allocNewConnection()
  in Football::Service<PianodConnection>.
- Connection is removed during fb_destroy_connection on connection,
  which is subsequent to the FB_EVENT_CLOSE.  This should not pose a problem.


Rx
--
Found issue in fb_accept_connection:
            service->connections [service->connection_count++] = connection;
Insertion of service into socket preceeded `if (fb_validate_connection) ...`.
If fb_validate_connection returned false, socket was closed and clean up done, but service's connection count not decremented,
leaving a rogue pointer in the service's connections.
Moved insertion to within `if (fb_validate_connection) ...`

Fix release in pianod2 r362


Version info:
pianod2 version 360
Build options:
  Switches: --enable-debug --with-compression
  SSL package: gnutls
  Sources:
    pandora
      curl compile version 7.74.0
      curl runtime version 7.74.0
    tonegenerator
    filesystem
  Media libraries:
    ffmpeg/libav (seems to be ffmpeg)
      avformat
        compile version 58.29.100
        runtime version 58.45.100
      avcodec
        compile version 58.54.100
        runtime version 58.91.100
      avfilter
        compile version 7.57.100
        runtime version 7.85.100
      avutil
        compile version 56.31.100
        runtime version 56.51.100
    taglib
      compile version 1.9.1
  Audio outputs:
    libao
    libavdevice
      compile version 58.8.100
      runtime version 58.10.100