count_coverage: ignore reads that have no SEQ field
count_coverage: ignore reads that have no SEQ field

Occasionally reads are encountered with SEQ = '*'. In this case,
`read.seq` is None and we must skip the `seq[qpos]` loop.
Fixes second instance of #603.

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