Fix segfault with marksweep-conc uninitialized ptr

I ran into a reproducable segfault when using marksweep-conc.
Initializing the cardtable pointer to NULL resolves the problem.

#6  sgen_los_update_cardtable_mod_union () at sgen-los.c:569
569					obj->cardtable_mod_union [i] |= start_card [i];

(gdb) p *obj
$46 = {next = 0x7f116bc01000, size = 281696, cardtable_mod_union = 0x1 <Address 0x1 out of bounds>, data = 0x7f116bc46000 ""}