Plug a mem leak in gdbus-connection test

Plug a mem leak in gdbus-connection test

==26538== 145 (24 direct, 121 indirect) bytes in 1 blocks are definitely lost in loss record 765 of 790
==26538==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==26538==    by 0x405233C: g_malloc (gmem.c:134)
==26538==    by 0x406A57E: g_slice_alloc (gslice.c:836)
==26538==    by 0x406A60C: g_slice_copy (gslice.c:858)
==26538==    by 0x4035C5A: g_error_copy (gerror.c:160)
==26538==    by 0x41B6387: g_simple_async_result_set_from_error (gsimpleasyncresult.c:638)
==26538==    by 0x41FCDEB: g_dbus_connection_call_done (gdbusconnection.c:4808)
==26538==    by 0x41B682E: g_simple_async_result_complete (gsimpleasyncresult.c:762)
==26538==    by 0x41B686A: complete_in_idle_cb (gsimpleasyncresult.c:772)
==26538==    by 0x404DA7C: g_idle_dispatch (gmain.c:4224)
==26538==    by 0x4049FCD: g_main_dispatch (gmain.c:2119)
==26538==    by 0x404B2C1: g_main_context_dispatch (gmain.c:2672)
==26538==    by 0x404B716: g_main_context_iterate (gmain.c:2750)
==26538==    by 0x404BE7F: g_main_loop_run (gmain.c:2958)
==26538==    by 0x804B5CC: test_connection_send (gdbus-connection.c:407)
==26538==    by 0x4073D04: test_case_run (gtestutils.c:1174)

Bug #627187.