ZeissZVIReader: ROI strings are UTF-16 little endian
ZeissZVIReader: Handle NULL labels

Also seek 4 bytes rather than 6 after the label, or else you end
up in missing the SourceTag and losing the correct position in the
stream.
ZeissZVIReader: Handle presence of Layer names

The layer name is typically absent, but some layers have names.
Cope with non-null layer names.
Merge pull request #471 from rleigh-dundee/zvi-roi-missing

ZVI ROI fixes