fix: check if the end row is greater than the start row (#1373)

## Description

Replica of #1372 to
main

#### PR checklist

- [x] Tests written/updated
- [ ] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)
- [ ] Updated relevant documentation (`docs/` or `spec/`) and code
comments

types/row_proof.go

@@ -23,7 +23,9 @@ type RowProof struct {
 // Validate performs checks on the fields of this RowProof. Returns an error if
 // the proof is not correctly constructed.
 func (rp RowProof) Validate() error {
-	// HACKHACK performing subtraction with unsigned integers is unsafe.
+	if rp.EndRow < rp.StartRow {
+		return fmt.Errorf("end row %d cannot be less than start row %d", rp.EndRow, rp.StartRow)
+	}
 	if int(rp.EndRow-rp.StartRow+1) != len(rp.RowRoots) {
 		return fmt.Errorf("the number of rows %d must equal the number of row roots %d", int(rp.EndRow-rp.StartRow+1), len(rp.RowRoots))
 	}

types/row_proof_test.go

@@ -41,6 +41,11 @@ func TestRowProofValidate(t *testing.T) {
 			rp:      validRowProof(),
 			wantErr: false,
 		},
+		{
+			name:    "start row greater than end row",
+			rp:      RowProof{StartRow: 10, EndRow: 5},
+			wantErr: true,
+		},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {