of: fdt: add missing allocation-failure check

The memory allocator passed to __unflatten_device_tree() (e.g. a wrapped
kzalloc) can fail so add the missing sanity check to avoid dereferencing
a NULL pointer.

Fixes: fe140423 ("of/flattree: Refactor unflatten_device_tree and add fdt_unflatten_tree")
Cc: stable <stable@vger.kernel.org>     # 2.6.38
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>