Check leaves size maximum in MerkleComputation

Belt and suspenders for future code changes.

Currently this function is only called from TransactionMerklePath() which sets leaves to the block transactions, so the Assume always holds.

src/consensus/merkle.cpp

@@ -4,6 +4,7 @@
 
 #include <consensus/merkle.h>
 #include <hash.h>
+#include <util/check.h>
 
 /*     WARNING! If you're reading this because you're learning about crypto
        and/or designing a new system that will use merkle trees, keep in mind
@@ -87,6 +88,7 @@ uint256 BlockWitnessMerkleRoot(const CBlock& block, bool* mutated)
 static void MerkleComputation(const std::vector<uint256>& leaves, uint256* proot, bool* pmutated, uint32_t leaf_pos, std::vector<uint256>* path)
 {
     if (path) path->clear();
+    Assume(leaves.size() <= UINT32_MAX);
     if (leaves.size() == 0) {
         if (pmutated) *pmutated = false;
         if (proot) *proot = uint256();