irc: limit size of data received from the server to prevent memory exhaustion

A malicious or compromised IRC server could send data with no end-of-line
(or a flood of "005" messages), making WeeChat accumulate it in a buffer
that grew without limit, until all memory was exhausted.

The unterminated received message and the accumulated "005" (ISUPPORT)
data are now bounded by IRC_SERVER_RECV_MSG_MAX_LENGTH and
IRC_SERVER_ISUPPORT_MAX_LENGTH: extra data is ignored once the limit is
reached.

src/plugins/irc/irc-server.c

@@ -3409,6 +3409,14 @@ irc_server_msgq_add_unterminated (struct t_irc_server *server,
 
     if (server->unterminated_message)
     {
+        /*
+         * limit the size of the unterminated message: once the maximum is
+         * reached, ignore the extra data (protection against a server sending
+         * a very long line without end-of-line, which would consume all the
+         * memory)
+         */
+        if (strlen (server->unterminated_message) >= IRC_SERVER_RECV_MSG_MAX_LENGTH)
+            return;
         unterminated_message2 =
             realloc (server->unterminated_message,
                      (strlen (server->unterminated_message) +