net: stop both net/net_processing before destroying them

This should avoid either attempting to use an invalid reference/pointer to the
other.

src/init.cpp

@@ -200,9 +200,13 @@ void Shutdown()
         pwalletMain->Flush(false);
 #endif
     MapPort(false);
+
+    // Because these depend on each-other, we make sure that neither can be
+    // using the other before destroying them.
     UnregisterValidationInterface(peerLogic.get());
-    g_connman.reset();
+    g_connman->Stop();
     peerLogic.reset();
+    g_connman.reset();
 
     StopTorControl();
     if (fDumpMempoolLater)