Remove null check from heap::deallocate

[nrc]

I added this to get DST landed. For ~[T] types I use a {ptr, len} fat pointer. If the vector is empy I use null for the pointer, so when this deallocates via drop glue jemalloc segfaults. It should be fairly easy to do a zero sized allocation or something and use that value and then we can remove the null check. That also allows us to use the Option to non-null pointer optimisation. For now I just want to get this branch landed though.

cc @thestinger, @alexcrichton

[huonw]

FWIW, Vec uses 1 as *T as the sentinel value for this sort of situation (to ensure that Option<...> still works, since that uses the "non-null" pointer as the discriminant).

[thestinger]

@nick29581: It seems that @luqmana landed the non-null pointer optimization for slices so this shouldn't be required anymore. Can it be closed?

[thestinger]

This ended up landing as part of the initial implementation, but I don't think it is actually accomplishing anything.