core: do not complain about low block rate if disconnected

In that case, we'll still keep the "Monero is now disconnected
from the network" near the end of the log

src/cryptonote_core/cryptonote_core.cpp

@@ -1833,7 +1833,7 @@ namespace cryptonote
   //-----------------------------------------------------------------------------------------------
   bool core::check_block_rate()
   {
-    if (m_offline || m_nettype == FAKECHAIN || m_target_blockchain_height > get_current_blockchain_height())
+    if (m_offline || m_nettype == FAKECHAIN || m_target_blockchain_height > get_current_blockchain_height() || m_target_blockchain_height == 0)
     {
       MDEBUG("Not checking block rate, offline or syncing");
       return true;