[libaccounts-qt] Fixed a few memory leaks

==5281==    at 0x4838AB0: malloc (vg_replace_malloc.c:270)
==5281==    by 0x6F41543: g_malloc (gmem.c:104)
==5281==    by 0x6F5DE1F: g_slice_alloc (gslice.c:1016)
==5281==    by 0x6F8801F: g_variant_new_from_children (gvariant-core.c:478)
==5281==    by 0x6F84CD3: g_variant_builder_end (gvariant.c:3654)
==5281==    by 0x6D0B543: ag_auth_data_get_login_parameters (ag-auth-data.c:383)
==5281==    by 0x4AF4383: Accounts::AuthData::parameters (auth-data.cpp:108)
==5281==    by 0x4A5238B: SSOSessionManager::createSsoIdentity (ssosessionmanager.cpp:154)