hdb: Fix crash when expected KR is missing

lib/hdb/common.c

@@ -995,6 +995,8 @@ derive_keys(krb5_context context,
 
         /* Installing keys invalidates `ckr', so we copy it */
         ret = hdb_entry_get_key_rotation(context, &h->entry, &ckr);
+        if (!ckr)
+            return ret;
         if (ret == 0)
             ret = copy_HDB_Ext_KeyRotation(ckr, &kr);
     }

lib/hdb/hdb.c

@@ -246,6 +246,9 @@ hdb_remove_base_keys(krb5_context context,
     size_t i, k;
 
     ret = hdb_entry_get_key_rotation(context, e, &ckr);
+    if (!ckr)
+        return 0;
+
     if (ret == 0) {
         /*
          * Changing the entry's extensions invalidates extensions obtained