superslab: reorder initialization

Setting kind and then immediately testing it seems wrong.

src/mem/superslab.h

@@ -82,12 +82,6 @@ namespace snmalloc
 
       if (kind != Super)
       {
-        // If this wasn't previously a Superslab, we need to set up the
-        // header.
-        kind = Super;
-        // Point head at the first non-short slab.
-        head = 1;
-
         if (kind != Fresh)
         {
           // If this wasn't previously Fresh, we need to zero some things.
@@ -97,6 +91,13 @@ namespace snmalloc
             new (&(meta[i])) Metaslab();
           }
         }
+
+        // If this wasn't previously a Superslab, we need to set up the
+        // header.
+        kind = Super;
+        // Point head at the first non-short slab.
+        head = 1;
+
 #ifndef NDEBUG
         auto curr = head;
         for (size_t i = 0; i < SLAB_COUNT - used - 1; i++)