wallet: Assert that enough was selected to cover the fees

When the fee is not subtracted from the outputs, the amount that has
been reserved for the fee (change_and_fee - change_amount) must be
enough to cover the fee that is needed. It would be a bug to not do so,
so use an assert to make this obvious if such a situation were to occur.

Author: achow101

src/wallet/spend.cpp

@@ -778,6 +778,10 @@ bool CWallet::CreateTransactionInternal(
         fee_needed = coin_selection_params.m_effective_feerate.GetFee(nBytes);
     }
 
+    // The only time that fee_needed should be less than the amount available for fees (in change_and_fee - change_amount) is when
+    // we are subtracting the fee from the outputs. If this occurs at any other time, it is a bug.
+    assert(coin_selection_params.m_subtract_fee_outputs || fee_needed <= change_and_fee - change_amount);
+
     // Update nFeeRet in case fee_needed changed due to dropping the change output
     if (fee_needed <= change_and_fee - change_amount) {
         nFeeRet = change_and_fee - change_amount;