Error check find_ram_offset

Spotted via code review, we initialize offset to 0 to avoid a
compiler warning, but in the unlikely case that offset is
never set to something else, we should abort instead of return
a value that will almost certainly cause problems.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
