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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/of/fdt.c

@@ -380,6 +380,9 @@ static void __unflatten_device_tree(void *blob,
 
 	/* Allocate memory for the expanded device tree */
 	mem = dt_alloc(size + 4, __alignof__(struct device_node));
+	if (!mem)
+		return NULL;
+
 	memset(mem, 0, size);
 
 	*(__be32 *)(mem + size) = cpu_to_be32(0xdeadbeef);