ns/stats: Add missing osmo_stat_item_group_free to gprs_nsvc_delete

The stat item group is not removed by gprs_nsvc_delete which will
corrupt the group list.

Addresses:
  valgrind tests/gbproxy/gbproxy_test
  [...]
  ==4541== Invalid write of size 4
  ==4541==    at 0x4071ACA: __llist_add (linuxlist.h:65)
  ==4541==    by 0x4071ACA: llist_add (linuxlist.h:81)
  ==4541==    by 0x4071ACA: osmo_stat_item_group_alloc
  (stat_item.c:112)
  ==4541==    by 0x407EDFD: gprs_nsvc_create (gprs_ns.c:244)
  ==4541==    by 0x408109D: gprs_ns_instantiate (gprs_ns.c:1388)
  ==4541==    by 0x804CFD3: test_gbproxy_ident_changes
  (gbproxy_test.c:1501)
  ==4541==    by 0x805FBD3: main (gbproxy_test.c:5803)

Sponsored-by: On-Waves ehf
1 file changed