test-alloc-util: add casts to bools from p ointers

test-alloc-util: add casts to bools from p ointers

C++03: "An rvalue of arithmetic, enumeration, pointer, or pointer to member
type can be converted to an rvalue of type bool. A zero value, null pointer
value, or null member pointer value is converted to false; any other value is
converted to true"

C should behave the same because pointers are scalars in C, but let's verify
that.