lsm/core: clear two-level data iterator before reload

If `_data_iter_fn` throws (e.g. a cloud-backed SST open fails),
`_data_iter` previously retained the iterator from the prior block,
leaving valid() reporting true against stale state. Null it before the
await so the iterator is observably invalid on a thrown future.

Author: ballard26

src/v/lsm/core/internal/two_level_iterator.cc

@@ -76,8 +76,10 @@ class impl : public iterator {
 
 private:
     ss::future<> init_data_block() {
+        // Clear previous iterator first in case `_data_iter_fn` throws.
+        _data_iter = nullptr;
+
         if (!_index_iter->valid()) {
-            _data_iter = nullptr;
             co_return;
         }
         auto handle = _index_iter->value();