usb-mtp: Return error on suspicious TYPE_DATA packet from initiator

CID 1390604
If the initiator sends a packet with TYPE_DATA set without
initiating a CMD_GET_OBJECT_INFO first, then usb_mtp_get_data
can trip on a null s->data_out.

Signed-off-by: Bandan Das <bsd@redhat.com>
Message-Id: <jpgr2m8ajfk.fsf_-_@linux.bootlegged.copy>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>