Improve check_bounds init check.

src/backend/backend.h

@@ -335,6 +335,12 @@ namespace snmalloc
         UNUSED(local_state);
         return concretePagemap.get_bounds();
       }
+
+      static bool is_initialised(LocalState* ls)
+      {
+        UNUSED(ls);
+        return concretePagemap.is_initialised();
+      }
     };
 
   private:

src/backend/pagemap.h

@@ -278,6 +278,14 @@ namespace snmalloc
         return body[p >> SHIFT];
     }
 
+    /**
+     * Check if the pagemap has been initialised.
+     */
+    [[nodiscard]] bool is_initialised() const
+    {
+      return body_opt != nullptr;
+    }
+
     /**
      * Return the starting address corresponding to a given entry within the
      * Pagemap. Also checks that the reference actually points to a valid entry.