of: unittest: fix memory leak in attach_node_and_children

In attach_node_and_children memory is allocated for full_name via
kasprintf. If the condition of the 1st if is not met the function
returns early without freeing the memory. Add a kfree() to fix that.

This has been detected with kmemleak:
Link: https://bugzilla.kernel.org/show_bug.cgi?id=205327



It looks like the leak was introduced by this commit:
Fixes: 5babefb7 ("of: unittest: allow base devicetree to have symbol metadata")

Signed-off-by: Erhard Furtner <erhard_f@mailbox.org>
Reviewed-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: Rob Herring <robh@kernel.org>