[test] Throw error instead of segfaulting in failure scenario

If the miner code is faulty and does not include any transactions in a block,
the code segfaults when it tries to access block transactions. Instead, add a
check that safely aborts the process.

src/test/miner_tests.cpp

@@ -123,6 +123,7 @@ void MinerTestingSetup::TestPackageSelection(const CChainParams& chainparams, co
     m_node.mempool->addUnchecked(entry.Fee(50000).Time(GetTime()).SpendsCoinbase(false).FromTx(tx));
 
     std::unique_ptr<CBlockTemplate> pblocktemplate = AssemblerForTest(chainparams).CreateNewBlock(scriptPubKey);
+    BOOST_REQUIRE_EQUAL(pblocktemplate->block.vtx.size(), 4);
     BOOST_CHECK(pblocktemplate->block.vtx[1]->GetHash() == hashParentTx);
     BOOST_CHECK(pblocktemplate->block.vtx[2]->GetHash() == hashHighFeeTx);
     BOOST_CHECK(pblocktemplate->block.vtx[3]->GetHash() == hashMediumFeeTx);
@@ -157,6 +158,7 @@ void MinerTestingSetup::TestPackageSelection(const CChainParams& chainparams, co
     hashLowFeeTx = tx.GetHash();
     m_node.mempool->addUnchecked(entry.Fee(feeToUse+2).FromTx(tx));
     pblocktemplate = AssemblerForTest(chainparams).CreateNewBlock(scriptPubKey);
+    BOOST_REQUIRE_EQUAL(pblocktemplate->block.vtx.size(), 6);
     BOOST_CHECK(pblocktemplate->block.vtx[4]->GetHash() == hashFreeTx);
     BOOST_CHECK(pblocktemplate->block.vtx[5]->GetHash() == hashLowFeeTx);
 
@@ -191,6 +193,7 @@ void MinerTestingSetup::TestPackageSelection(const CChainParams& chainparams, co
     tx.vout[0].nValue = 100000000 - 10000; // 10k satoshi fee
     m_node.mempool->addUnchecked(entry.Fee(10000).FromTx(tx));
     pblocktemplate = AssemblerForTest(chainparams).CreateNewBlock(scriptPubKey);
+    BOOST_REQUIRE_EQUAL(pblocktemplate->block.vtx.size(), 9);
     BOOST_CHECK(pblocktemplate->block.vtx[8]->GetHash() == hashLowFeeTx2);
 }