Fix case when an index contains no contigs; usually due to indexing a VCF/BCF file with no records.
Fix case when an index contains no contigs; usually due to indexing a VCF/BCF file with no records.

pysam-developers/pysam,kyleabeauchamp/pysam,bioinformed/pysam,pysam-developers/pysam,kyleabeauchamp/pysam,bioinformed/pysam,bioinformed/pysam,pysam-developers/pysam,bioinformed/pysam,pysam-developers/pysam,bioinformed/pysam,kyleabeauchamp/pysam,kyleabeauchamp/pysam,kyleabeauchamp/pysam