qmp: fix leak on callbacks that return both value and error

Direct leak of 4120 byte(s) in 1 object(s) allocated from:
    #0 0x7fa114931887 in __interceptor_calloc (/lib64/libasan.so.6+0xb0887)
    #1 0x7fa1144ad8f0 in g_malloc0 (/lib64/libglib-2.0.so.0+0x588f0)
    #2 0x561e3c9c8897 in qmp_object_add /home/elmarco/src/qemu/qom/qom-qmp-cmds.c:291
    #3 0x561e3cf48736 in qmp_dispatch /home/elmarco/src/qemu/qapi/qmp-dispatch.c:155
    #4 0x561e3c8efb36 in monitor_qmp_dispatch /home/elmarco/src/qemu/monitor/qmp.c:145
    qemu#5 0x561e3c8f09ed in monitor_qmp_bh_dispatcher /home/elmarco/src/qemu/monitor/qmp.c:234
    qemu#6 0x561e3d08c993 in aio_bh_call /home/elmarco/src/qemu/util/async.c:136
    qemu#7 0x561e3d08d0a5 in aio_bh_poll /home/elmarco/src/qemu/util/async.c:164
    qemu#8 0x561e3d0a535a in aio_dispatch /home/elmarco/src/qemu/util/aio-posix.c:380
    qemu#9 0x561e3d08e3ca in aio_ctx_dispatch /home/elmarco/src/qemu/util/async.c:298
    qemu#10 0x7fa1144a776e in g_main_context_dispatch (/lib64/libglib-2.0.so.0+0x5276e)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200325184723.2029630-3-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qapi/qmp-dispatch.c

@@ -155,6 +155,8 @@ QDict *qmp_dispatch(const QmpCommandList *cmds, QObject *request,
     cmd->fn(args, &ret, &err);
     qobject_unref(args);
     if (err) {
+        /* or assert(!ret) after reviewing all handlers: */
+        qobject_unref(ret);
         goto out;
     }