Fix errors when no boot state backend has been found

Current asserts on AumBootState object in aum_boot_state_*() functions ends up when there is no boot state backend available, i.e. for UEFI AMD64 targets, by trying to rollback the OS.
Replacing those asserts by tests returning default values allows AUM to handle this case gracefully and with better debug message.

Signed-off-by: Frédéric Danis frederic.danis@collabora.com