T or NIL.
The function pointerp returns true if ptr is a foreign
pointer and false otherwise.
In Allegro CL, foreign pointers are integers thus in this
implementation pointerp will return true for any ordinary integer.
CFFI> (foreign-alloc 32) => #<A Mac Pointer #x102D20> CFFI> (pointerp *) => T CFFI> (pointerp "this is not a pointer") => NIL