Fix shutdown in case of errors during initialization

PR bitcoin#10286 introduced a few steps which are not robust to early shutdown
in initialization.

Stumbled upon this with bitcoin#11781, not sure if there are other scenarios
that can trigger it, but it's harden against this in any case.

Author: laanwj

src/init.cpp

@@ -231,7 +231,10 @@ void PrepareShutdown()
 #endif
     StopMapPort();
 
-    UnregisterValidationInterface(peerLogic.get());
+    // Because these depend on each-other, we make sure that neither can be
+    // using the other before destroying them.
+    if (peerLogic) UnregisterValidationInterface(peerLogic.get());
+    if (g_connman) g_connman->Stop();
     peerLogic.reset();
     g_connman.reset();
 

src/validationinterface.cpp

@@ -57,7 +57,9 @@ void CMainSignals::UnregisterBackgroundSignalScheduler() {
 }
 
 void CMainSignals::FlushBackgroundCallbacks() {
-    m_internals->m_schedulerClient.EmptyQueue();
+    if (m_internals) {
+        m_internals->m_schedulerClient.EmptyQueue();
+    }
 }
 
 CMainSignals& GetMainSignals()
@@ -93,6 +95,9 @@ void UnregisterValidationInterface(CValidationInterface* pwalletIn)
 
 void UnregisterAllValidationInterfaces()
 {
+    if (!g_signals.m_internals) {
+        return;
+    }
     g_signals.m_internals->BlockFound.disconnect_all_slots();
     g_signals.m_internals->BlockChecked.disconnect_all_slots();
     g_signals.m_internals->Broadcast.disconnect_all_slots();