GT=. causes segmentation fault with RUTH

[jjfarrell]

When our VCF, has a GT=. instead of GT=./., a segmentation fault is triggered by ruth.

[E::bgzf_read_block] Failed to read BGZF block data at offset 1038687 expected 11606 bytes; hread returned 9871
[W::bgzf_read_block] EOF marker is absent. The input may be truncated