Fix crash if barcode isn't set

[philpem]

If a tape does not have a barcode set, my IBM drive throws a Check Condition status:

ERROR : Problem reading attribute 0806
SG_READ_ATT command error: SCSI status: Check Condition 
Fixed format, current; Sense key: Illegal Request
Additional sense: Invalid field in cdb
  Field replaceable unit code: 48
  Sense Key Specific: Error in Command: byte 8 bit 7
 Raw sense data (in hex), sb_len=32, embedded_len=96
        70 00 05 00 00 00 00 58  00 00 00 00 24 00 30 cf
        00 08 00 00 0e 00 20 20  20 20 20 20 20 00 00 00
        00 00 00 65 04 06 81 00  08 32 30 31 34 30 35 30
        37 04 07 80 00 08 00 00  00 00 00 00 40 00 04 08
        80 00 01 00 04 09 80 00  02 00 00 10 00 80 00 1c
        d7 59 04 24 47 36 41 42  41 58 31 39 46 55 4a 49

This PR handles this by setting the barcode buffer to an empty string and continuing, so mam-info can display the rest of the cartridge data.