Alpha 6.16: --haps all-homozygous segfault fix

This fixes a bug that could result in a segfault when the .pgen writer was told to expect phase and/or dosage data, all data was actually representable as unphased hardcalls, and the number of variants was a multiple of 32. The main context where this could actually happen was --haps on all-homozygous input; other import commands were not affected because they first scanned the input file to check whether phase/dosage data was actually present before initializing the .pgen writer.