wallet: Try estimating input size with external data if wallet fails

Instead of choosing whether to use the wallet or external data when
estimating the size of an input, first use the wallet, then try external
data if that failed.

src/wallet/spend.cpp

@@ -569,8 +569,12 @@ std::optional<SelectionResult> SelectCoins(const CWallet& wallet, CoinsResult& a
             if (!coin_control.GetExternalOutput(outpoint, txout)) {
                 return std::nullopt;
             }
+        }
+
+        if (input_bytes == -1) {
             input_bytes = CalculateMaximumSignedInputSize(txout, outpoint, &coin_control.m_external_provider, &coin_control);
         }
+
         // If available, override calculated size with coin control specified size
         if (coin_control.HasInputWeight(outpoint)) {
             input_bytes = GetVirtualTransactionSize(coin_control.GetInputWeight(outpoint), 0, 0);