also, check if the end is >= sector count so it'll always be
capped at disk->sectors - 1 value.
also when calculating disk free space, subtract
the alignment sector divisor. first partition
will always start at this value, so it is completely
unusable space.
Revert "also when calculating disk free space, subtract"

This reverts commit 5ebd889921a1a7972b017f1b56d192dded078251.
only subtract the alignment sector (equal to 1 mibibyte)
if there are no partitions yet defined. the first
partition always start here, so it's unusable space, but
only applies if there are no partitions yet.