Make GetFetchFlags always request witness objects from witness peers

This fixes a bug where we might (in exceedingly rare circumstances)
accidentally ban a node for sending us the first (potentially few)
segwit blocks in non-segwit mode.

src/main.cpp

@@ -4922,7 +4922,7 @@ void static ProcessGetData(CNode* pfrom, const Consensus::Params& consensusParam
 
 uint32_t GetFetchFlags(CNode* pfrom, CBlockIndex* pprev, const Consensus::Params& chainparams) {
     uint32_t nFetchFlags = 0;
-    if (IsWitnessEnabled(pprev, chainparams) && State(pfrom->GetId())->fHaveWitness) {
+    if ((pfrom->GetLocalServices() & NODE_WITNESS) && State(pfrom->GetId())->fHaveWitness) {
         nFetchFlags |= MSG_WITNESS_FLAG;
     }
     return nFetchFlags;