imsm: catch attempt to auto-layout zero-length arrays

imsm: catch attempt to auto-layout zero-length arrays

When -z is omitted reserve_space() looks to satisfy a zero length
allocation which lo and behold is equal to the amount of free space on a
full disk.  So, catch maxsize == 0 and simplify the return value from
merge_extents() to always equal amount of free space (no benefit to
having a special case ~0ULL == error).

Signed-off-by: Dan Williams <dan.j.williams@intel.com>