Plug a mem leak in network-address test

Plug a mem leak in network-address test

==4616== 46 (32 direct, 14 indirect) bytes in 1 blocks are definitely lost in loss record 193 of 305
==4616==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==4616==    by 0x4057094: g_malloc (gmem.c:134)
==4616==    by 0x406F2D6: g_slice_alloc (gslice.c:836)
==4616==    by 0x406F31B: g_slice_alloc0 (gslice.c:848)
==4616==    by 0x413D5BB: g_type_create_instance (gtype.c:1867)
==4616==    by 0x412372A: g_object_constructor (gobject.c:1482)
==4616==    by 0x4123147: g_object_newv (gobject.c:1347)
==4616==    by 0x41236BB: g_object_new_valist (gobject.c:1463)
==4616==    by 0x4122BB4: g_object_new (gobject.c:1181)
==4616==    by 0x41B2D0F: g_network_address_new (gnetworkaddress.c:262)
==4616==    by 0x8048A70: test_basic (network-address.c:10)

Bug #628331.