Assert that TaprootBuilder is Finalized during GetSpendData

GetSpendData needs to be finalized in order to be used. To avoid future
bugs, assert `!m_output_key.IsNull()` as m_output_key is only set during
Finalize.

Author: achow101

src/script/standard.cpp

@@ -485,6 +485,7 @@ WitnessV1Taproot TaprootBuilder::GetOutput() { return WitnessV1Taproot{m_output_
 TaprootSpendData TaprootBuilder::GetSpendData() const
 {
     assert(IsComplete());
+    assert(m_output_key.IsFullyValid());
     TaprootSpendData spd;
     spd.merkle_root = m_branch.size() == 0 ? uint256() : m_branch[0]->hash;
     spd.internal_key = m_internal_key;