Fix gcc warning message - Stylistic issue (Minor)

Warning used to be:
```
slablist_rem.c:1532:14: warning: assignment makes integer from pointer without a cast [enabled by default]
    rdl.sle_u = NULL;
```
This warning was raised by gcc without even using any warning flag like -Wall by default.
(GCC version 4.8.2)

The `sle_u` member of the `slablist_elem` union is of type uint64_t and is not used as a pointer type from my understanding of the code. GCC correctly raises a warning there. Changing the NULL with a zero makes more sense on this context.

References:
http://c-faq.com/null/nullor0.html