bad base64 causes early abort

[Habbie]

With reference to tobez/validns#69:

$  jdnssec-verifyzone minimal.com
org.xbill.DNS.Tokenizer$TokenizerException: minimal.com:5: invalid base64 encoding
	at org.xbill.DNS.Tokenizer.exception(Tokenizer.java:710)
	at org.xbill.DNS.Tokenizer.getBase64(Tokenizer.java:617)
	at org.xbill.DNS.Tokenizer.getBase64(Tokenizer.java:631)
	at org.xbill.DNS.SIGBase.rdataFromString(SIGBase.java:80)
	at org.xbill.DNS.Record.fromString(Record.java:472)
	at org.xbill.DNS.Master._nextRecord(Master.java:365)
	at org.xbill.DNS.Master.nextRecord(Master.java:388)
	at com.verisignlabs.dnssec.security.ZoneUtils.readZoneFile(ZoneUtils.java:74)
	at com.verisignlabs.dnssec.cl.VerifyZone.execute(VerifyZone.java:141)
	at com.verisignlabs.dnssec.cl.CLBase.run(CLBase.java:318)
	at com.verisignlabs.dnssec.cl.VerifyZone.main(VerifyZone.java:164)

verifyzone completely bails out on the base64 decoding failure, while it could theoretically still check the rest of the zone file.