Don't crash when FORMAT is set to the missing value (.)

It is not valid according to the spec, but issue #164 shows a VCF file
where the FORMAT column contains just a dot character. We have no way
of interpreting the subsequent genotype columns in that case, so this
patch ignores them.
Merge pull request #166 from martijnvermaat/format-none

Don't crash when FORMAT is set to the missing value (.)