free_list: Fix broken unittest

Fix compilation error (previously, silent dangling pointer and
double-free).

Author: CyberShadow

std/experimental/allocator/building_blocks/free_list.d

@@ -814,8 +814,9 @@ struct ContiguousFreeList(ParentAllocator,
     import std.experimental.allocator.building_blocks.region : Region;
     import std.experimental.allocator.gc_allocator : GCAllocator;
     import std.typecons : Ternary;
-    alias A = ContiguousFreeList!(Region!GCAllocator, 0, 64);
-    auto a = A(Region!GCAllocator(1024 * 4), 1024);
+    alias A = ContiguousFreeList!(Region!GCAllocator*, 0, 64);
+    auto r = Region!GCAllocator(1024 * 4);
+    auto a = A(&r, 1024);
 
     assert((() nothrow @safe @nogc => a.empty)() == Ternary.yes);