Fix allocator bug

test_allocator.cpp

@@ -0,0 +1,18 @@
+#include <stdio.h>
+#include "allocator.h"
+
+int main(int narg, char *args[])
+{
+    allocator a(8192);
+    for (int i = 0; i < 8192; i++)
+    {
+        uint64_t x = a.find_free();
+        if (x == UINT64_MAX)
+        {
+            printf("ran out of space %d\n", i);
+            return 1;
+        }
+        a.set(x, true);
+    }
+    return 0;
+}