Fix shutdown in case of errors during initialization

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

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

src/init.cpp

@@ -193,7 +193,7 @@ void Shutdown()
 
     // Because these depend on each-other, we make sure that neither can be
     // using the other before destroying them.
-    UnregisterValidationInterface(peerLogic.get());
+    if (peerLogic) UnregisterValidationInterface(peerLogic.get());
     if (g_connman) g_connman->Stop();
     peerLogic.reset();
     g_connman.reset();

src/validationinterface.cpp

@@ -49,8 +49,10 @@ void CMainSignals::UnregisterBackgroundSignalScheduler() {
 }
 
 void CMainSignals::FlushBackgroundCallbacks() {
+    if (m_internals) {
         m_internals->m_schedulerClient.EmptyQueue();
     }
+}
 
 void CMainSignals::RegisterWithMempoolSignals(CTxMemPool& pool) {
     pool.NotifyEntryRemoved.connect(boost::bind(&CMainSignals::MempoolEntryRemoved, this, _1, _2));
@@ -92,6 +94,9 @@ void UnregisterValidationInterface(CValidationInterface* pwalletIn) {
 }
 
 void UnregisterAllValidationInterfaces() {
+    if (!g_signals.m_internals) {
+        return;
+    }
     g_signals.m_internals->BlockChecked.disconnect_all_slots();
     g_signals.m_internals->Broadcast.disconnect_all_slots();
     g_signals.m_internals->Inventory.disconnect_all_slots();