of: unittest: fix memory leak in unittest_data_add

In unittest_data_add, a copy buffer is created via kmemdup. This buffer
is leaked if of_fdt_unflatten_tree fails. The release for the
unittest_data buffer is added.

Fixes: b951f9dc7f25 ("Enabling OF selftest to run without machine's devicetree")
Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
