added warning to NULL quality alignments in count_coverage
added warning to NULL quality alignments in count_coverage

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