Bugfix: division by zero in overlaps

[nakib103]

#1765

The issue should primarily be fixed by - Ensembl/ensembl-io#171
But this PR add checks for length to be zero and if so set the overlap percentage to be 100%. This can happen for old VCF format where INFO/SVLEN is missing and end position is calculated from INFO/END.

Test

input.vcf -

2	240692045	.	G	<DEL>	.	PASS	SVLEN=100

custom.vcf -

2       240692045       nssv211104      G       <DEL>   .       .       DBVARID=nssv211104;SVTYPE=DEL;END=240692045

CMD:

vep -i input.vcf --force --cache <cache_dir> --offline --cache_version 111 -a GRCh38 --custom file=custom.vcf,short_name=test,fields=DBVARID,num_records=all,reciprocal=1,format=vcf,coords=1,type=overlap,overlap_cutoff=0