power: sequencing: fix an invalid pointer dereference in error path

We may end up calling pwrseq_target_free() on a partially initialized
target object whose unit is either NULL or an ERR_PTR(). Avoid
dereferencing invalid memory by adding an appropriate check to
pwrseq_target_free().

Fixes: 249ebf3f

 ("power: sequencing: implement the pwrseq core")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/linux-pm/62a3531e-9927-40f8-b587-254a2dfa47ef@stanley.mountain/


Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20240712194004.241939-1-brgl@bgdev.pl


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>