of: unittest: Add of_node_put() before return

The local variable np in function of_unittest_platform_populate takes
the return value of of_find_node_by_path, which gets a node but does not
put it. If np is not put before return it may cause a memory leak. Hence
put np before a return statement.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>