dbghelp: Improved arena allocation in pool_alloc.

dlls/dbghelp/storage.c

@@ -108,13 +108,13 @@ void* pool_alloc(struct pool* pool, size_t len)
         }
     }
 
-    size = max( pool->arena_size, len + sizeof(struct pool_arena) );
-    arena = HeapAlloc(GetProcessHeap(), 0, size);
+    size = max( pool->arena_size, len );
+    arena = HeapAlloc(GetProcessHeap(), 0, size + sizeof(struct pool_arena));
     if (!arena) return NULL;
 
     ret = arena + 1;
     arena->current = (char*)ret + len;
-    arena->end = (char *)arena + size;
+    arena->end = (char*)ret + size;
     if (arena->current + 16 >= arena->end)
         list_add_tail( &pool->arena_full, &arena->entry );
     else