usb-redir: Fix crash on migration with no client connected

If no client is connected on the src side, then we won't receive a
parser during migrate, in this case usbredir_post_load() should be a nop,
rather then to try to derefefence the NULL dev->parser pointer.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
