Reinstate bam_iter_read(iter=NULL) reading the whole file

The old samtools API defined iter=NULL to mean to iterate over all
records with bam_read1(), which was assumed by bam_merge_core2() and
possibly third-party code.  Htslib iterators (currently) just segfault
with iter=NULL, so the previous simple shim accidentally broke this.

Fixes #83, though this will be refixed when the native htslib-API-based
samtools merge branch lands.