of: fdt: add missing allocation-failure check

commit 49e67dd17649b60b4d54966e18ec9c80198227f0 upstream.

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")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>