of: Fix NULL dereference in unflatten_and_copy()

Check whether initial_boot_params is NULL before dereferencing it in
unflatten_and_copy_device_tree() for the case where no device tree is
available but the arch can still boot to a minimal usable system without
it. In this case also log a warning for when the kernel log buffer is
obtainable.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>