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

+ Half-missing call (e.g. `1/.`) in `.vcf` file might cause SEGV in `vcf_to_bed` function;
    + when `bufptr[2]` equals to `'.'`, `ulii` gets huge (since `bufptr[2] - '0'` is negative).

+ Just ignore such calls, or it's also possible to log an error as `ped_to_bed_multichar_allele`, `lgen_to_bed` do.