Update Thread child id request timeout handling (ARMmbed#1715)

Do not use variable after deleting it.

Author: Arto Kinnunen

source/6LoWPAN/Thread/thread_host_bootstrap.c

@@ -1026,8 +1026,9 @@ static bool thread_child_id_req_timeout(int8_t interface_id, uint16_t msgId, boo
 
         entry_temp = mle_class_get_entry_by_ll64(cur->id, scanned_parent->linkMarginToParent, ll64, false, NULL);
         if (entry_temp) {
+            bool my_parent = thread_check_is_this_my_parent(cur, entry_temp);
             mle_class_remove_entry(cur->id, entry_temp);
-            if (thread_check_is_this_my_parent(cur, entry_temp)) {
+            if (my_parent) {
                 tr_debug("No parent resp - any-attach");
                 thread_bootstrap_connection_error(interface_id, CON_ERROR_NETWORK_ATTACH_FAIL, NULL);
                 goto exit;