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.

src/validationinterface.cpp

@@ -67,7 +67,9 @@ void CMainSignals::UnregisterBackgroundSignalScheduler() {
 }
 
 void CMainSignals::FlushBackgroundCallbacks() {
-    m_internals->m_schedulerClient.EmptyQueue();
+    if (m_internals) {
+        m_internals->m_schedulerClient.EmptyQueue();
+    }
 }
 
 CMainSignals& GetMainSignals()