Addresses US 79

[unique.ptr.single.ctor]/5 no longer requires for D not to be a pointer type. I believe this restriction was accidently removed when we relaxed the completeness reuqirements on T. The restriction needs to be put back in. Otherwise we have a run time failure that could have been caught at compile time:

{
unique_ptr<int, void(*)(void*)> p1(malloc(sizeof(int)));  // should not compile
}  // p1.~unique_ptr() dereferences a null function pointer
unique_ptr<int, void(*)(void*)> p2(malloc(sizeof(int)), free);  // ok