libfdisk: Fix assert error in free space handling

libfdisk: Fix assert error in free space handling

An off-by-one issue exists in fdisk_get_freespaces. It can trigger an
assert, as seen here:

$ dd if=/dev/zero of=cfdisk.iso bs=1M count=1
$ losetup -f cfdisk.iso
$ echo w | fdisk /dev/loop0
$ echo '1,2' | sfdisk /dev/loop0 --append
$ echo '3,' | sfdisk /dev/loop0 --append
$ sfdisk --list-free /dev/loop0
Aborted
$ _

Problem here is an invalid "grain" processing. A grain is considered
expected free space between partitions which can be required for proper
alignment. Normally, it's 1 MB but in this case our iso is merely 1 MB
so the grain is reduced to 1 byte.

The if-condition in question checks for "last + grain <= pa->start" and
therefore even triggers if there is no space between them (due to equal
check). Eventually, the start block address is higher than the end block
address which triggers the assert().

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>