Avoid potential coredump if pindex is NULL

src/wallet/wallet.cpp

@@ -2771,7 +2771,7 @@ int CWallet::ScanForWalletTransactions(CBlockIndex* pindexStart, bool fUpdate)
         // our wallet birthday (as adjusted for block time variability)
         while (pindex && nTimeFirstKey && (pindex->GetBlockTime() < (nTimeFirstKey - 7200))) {
             pindex = chainActive.Next(pindex);
-            pwalletMain->rescanHeight = pindex->GetHeight();
+            pwalletMain->rescanHeight = pindex ? pindex->GetHeight() : 0;
         }
 
         ShowProgress(_("Rescanning..."), 0); // show rescan progress in GUI as dialog or on splashscreen, if -rescan on startup