fix braindead linked list fail

I re-implemented a linked list for the slab freelist since we don't need to
manage the tail, check the previous item, and use it as a FIFO. However
prev/next must be managed so the slab mover is safe.

However I neglected to clear prev on a fetch, so if the slab mover was
zeroing the head of the freelist it would relink the next item in the freelist
with one in the main LRU.

Which results in chaos.

slabs.c

@@ -249,6 +249,7 @@ static void *do_slabs_alloc(const size_t size, unsigned int id) {
         /* return off our freelist */
         it = (item *)p->slots;
         p->slots = it->next;
+        if (it->next) it->next->prev = 0;
         p->sl_curr--;
         ret = (void *)it;
     } else {