Fix clang warning message (Minor)

clang using -Wall raises the following warnings:
```
...
slablist_rem.c:1295:10: warning: variable 'i' is uninitialized when used here [-Wuninitialized]
                while (i < s->s_elems) {

slablist_rem.c:1285:7: note: initialize the variable 'i' to silence this warning
        int i;

slablist_rem.c:1606:6: warning: variable 'stat' is used uninitialized whenever function 'slablist_xtract_simple' is called [-Wsometimes-uninitialized]
        int stat;

slablist_rem.c:1609:9: note: uninitialized use occurs here
        while (stat == 0 && (!first || !last)) {

slablist_rem.c:1606:10: note: initialize the variable 'stat' to silence this warning
        int stat;
...
```

As you can see it just bugs us to not assume that a newly declared variable is 0, even though that's the case with most modern compilers today.

This warning is in reference to http://c-faq.com/decl/initval.html

I doubt that slablists will ever be compiled with some old or unique compiler that won't zero out the variable during declaration. I just submitted this pull request to be on the safe side, so feel free to decline it.