He is right, here is debug info:
0x08095839 in clif_chsys_msg (channel=0xad5982b4, sd=0xa47f6020, msg=0xbf99139a "[ #trade ] normali : neviete nahodou kto tu vyvoal po meste MVP bosov? boli tu phonovia a je tu belzebub ") at clif.c:5538
5538 WFIFOHEAD(user->fd,msg_len + 12);
(gdb) print user->fd
$1 = -1
and guess what happens after calling session[-1]->wdata_size