CVS renato: fixed crash when a module is killed more than once

[FVWM CVS]

CVSROOT:        /home/cvs/fvwm
Module name:    fvwm
Changes by:     renato  07/08/15 09:27:51

Modified files:
        .              : ChangeLog 
        fvwm           : module_list.c 

Log message:
fixed crash when a module is killed more than once