[block encodings] Avoid fuzz blocking asserts in PartiallyDownloadedBlock

src/blockencodings.cpp

@@ -52,7 +52,8 @@ ReadStatus PartiallyDownloadedBlock::InitData(const CBlockHeaderAndShortTxIDs& c
     if (cmpctblock.shorttxids.size() + cmpctblock.prefilledtxn.size() > MAX_BLOCK_WEIGHT / MIN_SERIALIZABLE_TRANSACTION_WEIGHT)
         return READ_STATUS_INVALID;
 
-    assert(header.IsNull() && txn_available.empty());
+    if (!header.IsNull() || !txn_available.empty()) return READ_STATUS_INVALID;
+
     header = cmpctblock.header;
     txn_available.resize(cmpctblock.BlockTxCount());
 
@@ -167,14 +168,18 @@ ReadStatus PartiallyDownloadedBlock::InitData(const CBlockHeaderAndShortTxIDs& c
     return READ_STATUS_OK;
 }
 
-bool PartiallyDownloadedBlock::IsTxAvailable(size_t index) const {
-    assert(!header.IsNull());
+bool PartiallyDownloadedBlock::IsTxAvailable(size_t index) const
+{
+    if (header.IsNull()) return false;
+
     assert(index < txn_available.size());
     return txn_available[index] != nullptr;
 }
 
-ReadStatus PartiallyDownloadedBlock::FillBlock(CBlock& block, const std::vector<CTransactionRef>& vtx_missing) {
-    assert(!header.IsNull());
+ReadStatus PartiallyDownloadedBlock::FillBlock(CBlock& block, const std::vector<CTransactionRef>& vtx_missing)
+{
+    if (header.IsNull()) return READ_STATUS_INVALID;
+
     uint256 hash = header.GetHash();
     block = header;
     block.vtx.resize(txn_available.size());