Fix issue when clearing a Lst<Mixed> in an upgraded file (#7806)

Author: jedelbo

CHANGELOG.md

@@ -9,6 +9,7 @@
 * <How do the end-user experience this issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)
 * Valgrind could report a branch on an uninitialized read when opening something that is not an encrypted Realm file as an encrypted Realm file ([PR #7789](https://github.com/realm/realm-core/pull/7789), since v14.10.0).
 * Opening an FLX realm asynchronously may not wait to download all data ([#7720](https://github.com/realm/realm-core/issues/7720), since FLX sync was introduced).
+* Clearing a List of Mixed in an upgraded file would lead to an assertion failing ([#7771](https://github.com/realm/realm-core/issues/7771), since 14.0.0)
 * Fix compilation with Xcode 16 ([PR #7802](https://github.com/realm/realm-core/pull/7802))
 
 ### Breaking changes

src/realm/array_mixed.cpp

@@ -215,7 +215,9 @@ void ArrayMixed::clear()
     Array::set(payload_idx_int, 0);
     Array::set(payload_idx_pair, 0);
     Array::set(payload_idx_str, 0);
-    Array::set(payload_idx_ref, 0);
+    if (Array::size() > payload_idx_ref) {
+        Array::set(payload_idx_ref, 0);
+    }
     if (Array::size() > payload_idx_key) {
         if (auto ref = Array::get_as_ref(payload_idx_key)) {
             Array::destroy(ref, m_composite.get_alloc());