unique_ctrl_out is used in contexts where NULL may be a valid return, and is also used in contexts where NULL is not expected.

Improve the code by having two functions instead:
unique_ctrl_out_or_null: return the unique control out, or NULL if there is no or more than one control out.
unique_ctrl_out: return the unique control out, assert if there is no or more than one control out.

Keep the use of unique_ctrl_out where we expect the node to have exactly one control out. Change the use to unique_ctrl_out_or_null where also NULL iis an expected return value.