Fix memory leak in gVariantToQVariantMap

==26643== 78 bytes in 8 blocks are definitely lost in loss record 1,891 of 2,146
==26643==    at 0x4838AB0: malloc (vg_replace_malloc.c:270)
==26643==    by 0x6F45543: g_malloc (gmem.c:104)
==26643==    by 0x6F637DB: g_strdup (gstrfuncs.c:364)
==26643==    by 0x6F8A4AF: g_variant_valist_get (gvariant.c:4284)
==26643==    by 0x6F8A10F: g_variant_valist_get (gvariant.c:4670)
==26643==    by 0x6F8B89B: g_variant_iter_next (gvariant.c:5047)
==26643==    by 0x4AF80D7: Accounts::gVariantToQVariant(_GVariant*) (utils.cpp:39)
==26643==    by 0x4AF53EB: Accounts::AuthData::parameters() const (auth-data.cpp:111)