Protect bundle burn from adding assets with zero amount (#60)

Prevent the burning of assets with zero value.

src/builder.rs

@@ -346,6 +346,11 @@ impl Builder {
         if asset.is_native().into() {
             return Err("Burning is only possible for non-native assets");
         }
+
+        if value.inner() == 0 {
+            return Err("Burning is not possible for zero values");
+        }
+
         let cur = *self.burn.get(&asset).unwrap_or(&ValueSum::zero());
         let sum = (cur + value).ok_or("Orchard ValueSum operation overflowed")?;
         self.burn.insert(asset, sum);

tests/zsa.rs

@@ -502,4 +502,21 @@ fn zsa_issue_and_transfer() {
         Ok(_) => panic!("Test should fail"),
         Err(error) => assert_eq!(error, "Burning is only possible for non-native assets"),
     }
+
+    // 12. Try to burn zero value - should fail
+    let result = build_and_verify_bundle(
+        vec![&zsa_spend_1],
+        vec![TestOutputInfo {
+            value: zsa_spend_1.note.value(),
+            asset: zsa_spend_1.note.asset(),
+        }],
+        vec![(zsa_spend_1.note.asset(), NoteValue::from_raw(0))],
+        anchor,
+        2,
+        &keys,
+    );
+    match result {
+        Ok(_) => panic!("Test should fail"),
+        Err(error) => assert_eq!(error, "Burning is not possible for zero values"),
+    }
 }