test: add missing BOOST_ASSERT

this was missed in the original PR

Author: josibake

src/wallet/test/availablecoins_tests.cpp

@@ -89,6 +89,7 @@ BOOST_FIXTURE_TEST_CASE(BasicOutputTypesTest, AvailableCoinsTestingSetup)
 
     // P2SH-SEGWIT
     dest = wallet->GetNewDestination(OutputType::P2SH_SEGWIT, "");
+    BOOST_ASSERT(dest);
     AddTx(CRecipient{{GetScriptForDestination(*dest)}, 3 * COIN, /*fSubtractFeeFromAmount=*/true});
     available_coins = AvailableCoins(*wallet);
     BOOST_CHECK_EQUAL(available_coins.coins[OutputType::P2SH_SEGWIT].size(), 2U);