Check m_internals in UnregisterValidationInterface

When a wallet is created it is registered in the validation interface (in
CWallet::CreateWalletFromFile) but it is not immediately added to the
wallets list. If a shutdown is requested before AddWallet (case more
evident when -rescan is set) then m_internals can be released (in
Shutdown -> UnregisterBackgroundSignalScheduler) before the wallet and
then ReleaseWallet would call UnregisterValidationInterface with
m_internals already released.

src/validationinterface.cpp

@@ -107,7 +107,9 @@ void RegisterValidationInterface(CValidationInterface* pwalletIn) {
 }
 
 void UnregisterValidationInterface(CValidationInterface* pwalletIn) {
-    g_signals.m_internals->m_connMainSignals.erase(pwalletIn);
+    if (g_signals.m_internals) {
+        g_signals.m_internals->m_connMainSignals.erase(pwalletIn);
+    }
 }
 
 void UnregisterAllValidationInterfaces() {