net: fix a few cases where messages were sent rather than dropped upon disconnection

75ead758 turned these into crashes in the event of a handshake failure, most
notably when a peer does not offer the expected services.

There are likely other cases that these assertions will find as well.

src/main.cpp

@@ -6351,7 +6351,7 @@ bool SendMessages(CNode* pto, CConnman& connman)
             // Ping automatically sent as a latency probe & keepalive.
             pingSend = true;
         }
-        if (pingSend) {
+        if (pingSend && !pto->fDisconnect) {
             uint64_t nonce = 0;
             while (nonce == 0) {
                 GetRandBytes((unsigned char*)&nonce, sizeof(nonce));
@@ -6432,7 +6432,7 @@ bool SendMessages(CNode* pto, CConnman& connman)
         if (pindexBestHeader == NULL)
             pindexBestHeader = chainActive.Tip();
         bool fFetch = state.fPreferredDownload || (nPreferredDownload == 0 && !pto->fClient && !pto->fOneShot); // Download if this is a nice peer, or we have no nice peers and this one might do.
-        if (!state.fSyncStarted && !pto->fClient && !fImporting && !fReindex) {
+        if (!state.fSyncStarted && !pto->fClient && !pto->fDisconnect && !fImporting && !fReindex) {
             // Only actively request headers from a single peer, unless we're close to today.
             if ((nSyncStarted == 0 && fFetch) || pindexBestHeader->GetBlockTime() > GetAdjustedTime() - 24 * 60 * 60) {
                 state.fSyncStarted = true;