memory leak: test_group_readwrite leaks an attrib

- groups were not clearing attribs when they were destroyed.
- new critbit module fixes leak in cb_erase

critbit

@@ -1 +1 @@
-Subproject commit 934c2dd94d41da19637a76a1a8b3dfeb7aa8524d
+Subproject commit ee7a32b9b8986bf2ea6b287975f9ef0ec997b7a3

src/kernel/group.c

@@ -132,6 +132,9 @@ void free_group(group * g)
     assert(*g_ptr == g);
     *g_ptr = g->nexthash;
 
+    if (g->attribs) {
+        a_removeall(&g->attribs, NULL);
+    }
     while (g->allies) {
         ally *a = g->allies;
         g->allies = a->next;