Ignore getheaders requests when not synced.

Rebased-From: a1ba0778dd
Github-Pull: #6172

src/main.cpp

@@ -4255,6 +4255,9 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
 
         LOCK(cs_main);
 
+        if (IsInitialBlockDownload())
+            return true;
+
         CBlockIndex* pindex = NULL;
         if (locator.IsNull())
         {