Fix register_clean_up being called with ScopedAllocator. (#344)

src/mem/scopedalloc.h

@@ -25,7 +25,10 @@ namespace snmalloc
     /**
      * Constructor.  Claims an allocator from the global pool
      */
-    ScopedAllocator() = default;
+    ScopedAllocator()
+    {
+      alloc.init();
+    };
 
     /**
      * Copying is not supported, it could easily lead to accidental sharing of

src/test/func/thread_alloc_external/thread_alloc_external.cc

@@ -37,4 +37,15 @@ int main()
   }
 
   ThreadAlloc::get().teardown();
+
+  // This checks that the scoped allocator does not call
+  // register clean up, as this configuration will fault
+  // if that occurs.
+  auto a2 = ScopedAllocator();
+  for (size_t i = 0; i < 1000; i++)
+  {
+    auto r1 = a2->alloc(i);
+
+    a2->dealloc(r1);
+  }
 }