hapLrt: fix segfault when accessing genotype field.
hapLrt: fix segfault when accessing genotype field.

When the target is not present in the vcf file, the target
map was empty. An illegal value was then accessed when converting
the string value to a double in the method unphred.

vcflib/vcflib,vcflib/vcflib,vcflib/vcflib,vcflib/vcflib,ekg/vcflib,ekg/vcflib,vcflib/vcflib,vcflib/vcflib,ekg/vcflib,ekg/vcflib,ekg/vcflib,vcflib/vcflib,vcflib/vcflib