Plug a mem leak in gdbus-connection test

Plug a mem leak in gdbus-connection test

==25403== 49 (24 direct, 25 indirect) bytes in 1 blocks are definitely lost in loss record 603 of 787
==25403==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==25403==    by 0x405233C: g_malloc (gmem.c:134)
==25403==    by 0x406A57E: g_slice_alloc (gslice.c:836)
==25403==    by 0x406A5C3: g_slice_alloc0 (gslice.c:848)
==25403==    by 0x4035B4E: g_error_new_literal (gerror.c:117)
==25403==    by 0x4035ED9: g_set_error_literal (gerror.c:314)
==25403==    by 0x41F6434: g_dbus_connection_close_sync (gdbusconnection.c:1284)
==25403==    by 0x804A861: test_connection_life_cycle (gdbus-connection.c:158)
==25403==    by 0x4073D04: test_case_run (gtestutils.c:1174)
==25403==    by 0x4073FC2: g_test_run_suite_internal (gtestutils.c:1223)
==25403==    by 0x4074077: g_test_run_suite_internal (gtestutils.c:1233)
==25403==    by 0x4074077: g_test_run_suite_internal (gtestutils.c:1233)
==25403==    by 0x40741FB: g_test_run_suite (gtestutils.c:1274)
==25403==    by 0x40733E5: g_test_run (gtestutils.c:877)
==25403==    by 0x804DC92: main (gdbus-connection.c:1024)

Bug #627187.