Should update POS on unmapped reads in 'samtools depad'

Should update POS on unmapped reads in 'samtools depad'

Previously unmapped reads with a POS entry (e.g. those reads with
a mate which was mapped) were not being updated.

This has passed my testing with some non-trivial de novo MIRA 3.9
assemblies, coupled with the new 'samtools index' BIN verification.