net: Add missing lock in ProcessHeadersMessage(...)

Reading the variable mapBlockIndex requires holding the mutex cs_main.

The new "Disconnect outbound peers relaying invalid headers" code
added in commit 37886d5e2f and merged
as part of #11568 two days ago did not lock cs_main prior to accessing
mapBlockIndex.

src/net_processing.cpp

@@ -1262,8 +1262,8 @@ bool static ProcessHeadersMessage(CNode *pfrom, CConnman *connman, const std::ve
     if (!ProcessNewBlockHeaders(headers, state, chainparams, &pindexLast, &first_invalid_header)) {
         int nDoS;
         if (state.IsInvalid(nDoS)) {
+            LOCK(cs_main);
             if (nDoS > 0) {
-                LOCK(cs_main);
                 Misbehaving(pfrom->GetId(), nDoS);
             }
             if (punish_duplicate_invalid && mapBlockIndex.find(first_invalid_header.GetHash()) != mapBlockIndex.end()) {