Address a couple of feedback items from @Tapanito and @shawnxie999:

- Call verifyDepositAuth in VaultWithdraw and LoanBrokerCoverWithdraw to
  a destination.
- Update a couple of impossible error returns to log a message, but
  exclude from coverage.
- Fail if the LoanBroker.LoanSequence overflows to 0.

src/xrpld/app/tx/detail/LoanBrokerCoverWithdraw.cpp

@@ -3,6 +3,8 @@
 #include <xrpld/app/misc/LendingHelpers.h>
 #include <xrpld/app/tx/detail/Payment.h>
 
+#include <xrpl/ledger/CredentialHelpers.h>
+
 namespace ripple {
 
 bool
@@ -59,7 +61,12 @@ LoanBrokerCoverWithdraw::preclaim(PreclaimContext const& ctx)
     }
     auto const vault = ctx.view.read(keylet::vault(sleBroker->at(sfVaultID)));
     if (!vault)
-        return tefBAD_LEDGER;  // LCOV_EXCL_LINE
+    {
+        // LCOV_EXCL_START
+        JLOG(ctx.j.fatal()) << "Vault is missing for Broker " << brokerID;
+        return tefBAD_LEDGER;
+        // LCOV_EXCL_STOP
+    }
 
     auto const vaultAsset = vault->at(sfAsset);
     if (amount.asset() != vaultAsset)
@@ -158,6 +165,13 @@ LoanBrokerCoverWithdraw::doApply()
             !isTesSuccess(ter) && ter != tecDUPLICATE)
             return ter;
     }
+    else
+    {
+        auto dstSle = view().peek(keylet::account(dstAcct));
+        if (auto err =
+                verifyDepositPreauth(tx, view(), account_, dstAcct, dstSle, j_))
+            return err;
+    }
 
     // Sanity check
     if (accountHolds(

src/xrpld/app/tx/detail/LoanSet.cpp

@@ -609,6 +609,10 @@ LoanSet::doApply()
     // and is distinct from the broker's pseudo-account's owner count
     adjustOwnerCount(view, brokerSle, 1, j_);
     loanSequenceProxy += 1;
+    // The sequence should be extremely unlikely to roll over, but fail if it
+    // does
+    if (loanSequenceProxy == 0)
+        return tecMAX_SEQUENCE_REACHED;
     view.update(brokerSle);
 
     // Put the loan into the pseudo-account's directory

src/xrpld/app/tx/detail/VaultWithdraw.cpp

@@ -245,6 +245,13 @@ VaultWithdraw::doApply()
             !isTesSuccess(ter) && ter != tecDUPLICATE)
             return ter;
     }
+    else
+    {
+        auto dstSle = view().peek(keylet::account(dstAcct));
+        if (auto err = verifyDepositPreauth(
+                ctx_.tx, view(), account_, dstAcct, dstSle, j_))
+            return err;
+    }
 
     // Transfer assets from vault to depositor or destination account.
     if (auto const ter = accountSend(