Rework free list so that 0 is the placeholder.

This is needed because in some configurations the constructor for the
global placeholder is not called before the first allocation (i.e. when
other globals call the allocator in their constructor) and so we ended
up following a null pointer.

src/mem/alloc.h

@@ -1072,7 +1072,7 @@ namespace snmalloc
       assert(sizeclass < NUM_SMALL_CLASSES);
       auto& fl = small_fast_free_lists[sizeclass];
       auto head = fl.value;
-      if (likely((reinterpret_cast<size_t>(head) & 1) == 0))
+      if (likely(head != nullptr))
       {
         void* p = head;
         // Read the next slot from the memory that's about to be allocated.