of: fdt: add missing allocation-failure check

commit 49e67dd1 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>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/of/fdt.c

@@ -388,6 +388,8 @@ static void __unflatten_device_tree(struct boot_param_header *blob,
 	/* Allocate memory for the expanded device tree */
 	mem = (unsigned long)
 		dt_alloc(size + 4, __alignof__(struct device_node));
+	if (!mem)
+		return NULL;
 
 	memset((void *)mem, 0, size);