qdev-monitor: fix segmentation fault on qdev_device_help()

Normally, qmp_device_list_properties() may return NULL when
a device haven't special properties excpet Object and DeviceState
properties, such as virtio-balloon-device.

We just need check local_err instead of prop_list.

Example:

Segmentation fault (core dumped)

The backtrace as below:

Program received signal SIGSEGV, Segmentation fault.
0x00005555559af1a8 in error_get_pretty (err=0x0) at util/error.c:152
152         return err->msg;
(gdb) bt
#0  0x00005555559af1a8 in error_get_pretty (err=0x0) at util/error.c:152
#1  0x000055555572fce9 in qdev_device_help (opts=0x5555562fdfe0) at qdev-monitor.c:210
#2  0x000055555574a6f2 in device_help_func (opts=0x5555562fdfe0, opaque=0x0) at vl.c:2362
#3  0x00005555559c0a33 in qemu_opts_foreach (list=0x555555dd0b40 <qemu_device_opts>,
    func=0x55555574a6ca <device_help_func>, opaque=0x0, abort_on_failure=0) at util/qemu-option.c:1072
#4  0x000055555574f514 in main (argc=3, argv=0x7fffffffe218, envp=0x7fffffffe238) at vl.c:4246

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>