SocketServer: close _serverFd exactly once (#585)

SocketServer::stop() closed _serverFd but never cleared it, and stop()
runs again from both ~WebSocketServer() and ~SocketServer(), so every
teardown closed the same fd number two or three times. If another
thread opens a descriptor between those closes, the kernel reuses the
number and the stale close destroys it — observed as intermittent
process aborts ("Unexpected error 9 on netlink descriptor", glibc's
getaddrinfo netlink probe) when a DNS lookup raced a server teardown.

Close the fd once and set it to -1, in stop() and in the listen()
error paths.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: zardus

ixwebsocket/IXSocketServer.cpp

@@ -95,6 +95,7 @@ namespace ix
                << "at address " << _host << ":" << _port << " : " << strerror(Socket::getErrno());
 
             Socket::closeSocket(_serverFd);
+            _serverFd = -1;
             return std::make_pair(false, ss.str());
         }
 
@@ -113,6 +114,7 @@ namespace ix
                    << strerror(Socket::getErrno());
 
                 Socket::closeSocket(_serverFd);
+                _serverFd = -1;
                 return std::make_pair(false, ss.str());
             }
 
@@ -125,6 +127,7 @@ namespace ix
                    << strerror(Socket::getErrno());
 
                 Socket::closeSocket(_serverFd);
+                _serverFd = -1;
                 return std::make_pair(false, ss.str());
             }
         }
@@ -143,6 +146,7 @@ namespace ix
                    << strerror(Socket::getErrno());
 
                 Socket::closeSocket(_serverFd);
+                _serverFd = -1;
                 return std::make_pair(false, ss.str());
             }
 
@@ -155,6 +159,7 @@ namespace ix
                    << strerror(Socket::getErrno());
 
                 Socket::closeSocket(_serverFd);
+                _serverFd = -1;
                 return std::make_pair(false, ss.str());
             }
         }
@@ -169,6 +174,7 @@ namespace ix
                << "at address " << _host << ":" << _port << " : " << strerror(Socket::getErrno());
 
             Socket::closeSocket(_serverFd);
+            _serverFd = -1;
             return std::make_pair(false, ss.str());
         }
 
@@ -231,7 +237,16 @@ namespace ix
         }
 
         _conditionVariable.notify_one();
-        Socket::closeSocket(_serverFd);
+
+        // stop() runs again from ~WebSocketServer() and ~SocketServer(), so
+        // close the listening fd exactly once: a second close of the stale
+        // number would destroy whatever descriptor another thread has since
+        // opened with it.
+        if (_serverFd != -1)
+        {
+            Socket::closeSocket(_serverFd);
+            _serverFd = -1;
+        }
     }
 
     void SocketServer::setConnectionStateFactory(