of: unittest: Unlock on error in unittest_data_add()

The of_overlay_mutex_unlock() was accidentally deleted if "of_root" is
NULL.  Change this to a goto unlock.

Fixes: d1eabd21 ("of: unittest: treat missing of_root as error instead of fixing up")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/aBHZ1DvXiBcZkWmk@stanley.mountain


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>