Kaartic Sivaraam [off-list ref] writes:

There was a usage for which there's no compelling reason.So, replace
such a usage as with something else that expresses the intent more
clearly.

I actually think this is a good example of the exception-rule.  The
function wants to take true or false in "int", and the caller has a
pointer.  And !!ptr is a shorter and more established way than ptr
!= NULL to turn non-NULL ness into an int boolean, without having to
either repeat or introducing an otherwise unnecessary temporary.