p2p: restore safe sync mode when target height drops

(cherry picked from commit 12098ff5d66d86d73360d28c548c350f6e7b64c8)

src/cryptonote_protocol/cryptonote_protocol_handler.inl

@@ -2860,6 +2860,8 @@ skip:
     {
       MINFO("Target height decreasing from " << previous_target << " to " << target);
       m_core.set_target_blockchain_height(target);
+      if (target < m_core.get_current_blockchain_height() + 5)
+        m_core.safesyncmode(true);
       if (target == 0 && context.m_state > cryptonote_connection_context::state_before_handshake && !m_stopping)
       {
         MCWARNING("global", "wownerod is now disconnected from the network");