dpll: sanitize possible null pointer dereference in dpll_pin_parent_pin_set()

dpll: sanitize possible null pointer dereference in dpll_pin_parent_pin_set()

User may not pass DPLL_A_PIN_STATE attribute in the pin set operation
message. Sanitize that by checking if the attr pointer is not null
and process the passed state attribute value only in that case.

Reported-by: Xingyuan Mo <hdthky0@gmail.com>
Fixes: 9d71b54b65b1 ("dpll: netlink: Add DPLL framework base functions")
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://lore.kernel.org/r/20231211083758.1082853-1-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>