Avoid overflow on empty multiproof (#4564)

Author: frangio

.changeset/large-humans-remain.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': patch
+---
+
+`MerkleProof`: Use custom error to report invalid multiproof instead of reverting with overflow panic.

contracts/utils/cryptography/MerkleProof.sol

@@ -118,7 +118,7 @@ library MerkleProof {
         uint256 totalHashes = proofFlags.length;
 
         // Check proof validity.
-        if (leavesLen + proofLen - 1 != totalHashes) {
+        if (leavesLen + proofLen != totalHashes + 1) {
             revert MerkleProofInvalidMultiproof();
         }
 
@@ -174,7 +174,7 @@ library MerkleProof {
         uint256 totalHashes = proofFlags.length;
 
         // Check proof validity.
-        if (leavesLen + proofLen - 1 != totalHashes) {
+        if (leavesLen + proofLen != totalHashes + 1) {
             revert MerkleProofInvalidMultiproof();
         }