initialize store_t::name

the field is marked foreign (for the drivers), so a recycled store may
contain an old pointer in it. that would make our error path crash.

REFMAIL: CAF_KswU7aBS7unnK+rdZy1PG_8SZUAW=tcg75HixDLLE0w3Lhw@mail.gmail.com

src/sync.c

@@ -598,6 +598,7 @@ sync_boxes( store_t *ctx[], const char *names[], channel_conf_t *chan,
 	svars->uidval[0] = svars->uidval[1] = -1;
 	svars->srecadd = &svars->srecs;
 
+	ctx[0]->name = ctx[1]->name = 0;
 	for (t = 0; t < 2; t++) {
 		ctx[t]->orig_name =
 			(!names[t] || (ctx[t]->conf->map_inbox && !strcmp( ctx[t]->conf->map_inbox, names[t] ))) ?