Fix SEGV in vcf_to_bed: Ignore half-missing call in .vcf file.

This reverts commit b451aa29fddf63b80a8ad42fa6eb59df826a7621.
Merge pull request #6 from mkanai/half_missing_call_in_vcf

Fix SEGV in vcf_to_bed: Ignore half-missing call in .vcf file.