qom: Don't keep error value between object_property_parse() calls

qom: Don't keep error value between object_property_parse() calls

When handling errp==NULL at object_apply_global_props(), we are
leaving the old error value in `err` after printing a warning.
This makes QEMU crash if two global properties generate warnings:

  $ echo device_add rtl8139 | qemu-system-x86_64 -monitor stdio -global rtl8139.xxx=yyy -global rtl8139.xxx=zzz
  warning: can't apply global rtl8139.xxx=yyy: Property '.xxx' not found
  qemu-system-x86_64: util/error.c:57: error_setv: Assertion `*errp == NULL' failed.
  Aborted (core dumped)

Fix that by making `err` go out of scope immediately after the
warn_report_err() call.

Fixes: 50545b2cc029 "qdev-props: call object_apply_global_props()"
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190110020259.8492-2-ehabkost@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>