liblp: Handle invalid alignment offsets correctly.

When stacking devices with dm-linear, it is possible to get a -1
alignment offset. In this case we should set alignment_offset to 0
rather than fail later trying to divide -1 by the sector size.

Bug: 134536978
Test: manual test
Change-Id: I3862fdda69531a3b230b7316707469a49ba871d6