Core/Sockets: Always try to send MSG_NOSIGNAL in peer().send()

Fixes RASocket::authenticate crash

"MSG_NOSIGNAL:
If you send() to a remote host which is no longer recv()ing, you'll typically get the signal SIGPIPE. Adding this flag prevents that signal from being raised."

Closes #5040
Thanks to @derex for the hint

src/server/authserver/Server/RealmSocket.cpp

@@ -138,7 +138,11 @@ ssize_t RealmSocket::noblk_send(ACE_Message_Block &message_block)
         return -1;
 
     // Try to send the message directly.
+#ifdef MSG_NOSIGNAL
     ssize_t n = peer().send(message_block.rd_ptr(), len, MSG_NOSIGNAL);
+#else
+    ssize_t n = peer().send(message_block.rd_ptr(), len);
+#endif // MSG_NOSIGNAL
 
     if (n < 0)
     {