prevent multiply with overflow resulting in odd errors/segv

src/macros.rs

@@ -472,7 +472,9 @@ macro_rules! resource_error_call_result {
 
 macro_rules! heap_index {
     ($idx:expr) => {
-        ($idx) * std::mem::size_of::<HeapCellValue>()
+        std::mem::size_of::<HeapCellValue>()
+            .checked_mul($idx)
+            .unwrap()
     };
 }