I'm sorry, I was not clear enough. Tk_CreateOptionTable() (line 1981) will be called during the creation of the text widget (for "sel" tag). Then Tcl_CreateHashEntry() (line 1924) will be called during "tag configure big -font {Helvetica -24}", after Tk_CreateOptionTable() has been called the first time.

BTW, probably it helps if you link with a Tcl library which is compiled with stack protector (-fstack-protector-all).