Plug a mem leak in the gdbus-proxy test

Plug a mem leak in the gdbus-proxy test

==23341== 85 (24 direct, 61 indirect) bytes in 1 blocks are definitely lost in loss record 900 of 971
==23341==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==23341==    by 0x4057094: g_malloc (gmem.c:134)
==23341==    by 0x406F2D6: g_slice_alloc (gslice.c:836)
==23341==    by 0x406F31B: g_slice_alloc0 (gslice.c:848)
==23341==    by 0x403A751: g_error_new_valist (gerror.c:54)
==23341==    by 0x403AAD4: g_set_error (gerror.c:240)
==23341==    by 0x420B807: decode_method_reply (gdbusconnection.c:4774)
==23341==    by 0x420C2BA: g_dbus_connection_call_sync (gdbusconnection.c:5188)
==23341==    by 0x421B7C9: g_dbus_proxy_call_sync (gdbusproxy.c:2477)
==23341==    by 0x804BD89: test_bogus_method_return (gdbus-proxy.c:430)

Bug #628331.