btape: dumplabel only when label is valid

[bruno-at-bareos]

This PR will fix the following IOT trap when btape readlable is used with blank tape, by allowing dumplabel only for valid label case.

*readlabel
btape: stored/btape.cc:530-0 Volume has no label.

Volume Label:
Id                : **error**VerNo             : 0
VolName           :
PrevVolName       :
VolFile           : 0
LabelType         : Unknown 0
LabelSize         : 0
PoolName          :
MediaType         :
PoolType          :
HostName          :
14-sep 15:08 btape: ERROR in stored/label.cc:860 Failed ASSERT: dev->VolHdr.VerNum >= 11
btape: stored/label.cc:860-0 Failed ASSERT: dev->VolHdr.VerNum >= 11
BAREOS interrupted by signal 6: IOT trap
btape, btape got signal 6 - IOT trap. Attempting traceback.
exepath=/usr/sbin/
Calling: /usr/sbin/btraceback /usr/sbin/btape 13788 /tmp
bsmtp: tools/bsmtp.cc:467-0 Failed to connect to mailhost localhost
The btraceback call returned 1
Dumping: /tmp/btape.13788.bactrace

 cat /tmp/btape.13788.bactrace
Attempt to dump current JCRs. njcrs=1
threadid=0xc000007f246b0277 JobId=0 JobStatus=T jcr=0x16e08a0 name=btape
threadid=0xc000007f246b0277 killable=0 JobId=0 JobStatus=T jcr=0x16e08a0 name=btape
        UseCount=1
        JobType=U JobLevel=F
        sched_time=14-sep-2022 15:07 start_time=01-jan-1970 01:00
        end_time=01-jan-1970 01:00 wait_time=01-jan-1970 01:00
        db=(nil) db_batch=(nil) batch_started=0

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[pstorz]

Looks good to me

[pstorz]

Good work!