fix: handle invalid index in centerItem and return null for out-of-range subtitle indices

shared/src/commonMain/kotlin/fr/ajaury/gwenedeg/player/ui/SubtitleList.kt

@@ -98,6 +98,8 @@ fun SubtitleList(
 }
 
 private suspend fun LazyListState.centerItem(index: Int) {
+    if (index < 0 || index >= this.layoutInfo.totalItemsCount) return
+
     val info = this.layoutInfo
     val viewportCenter = (info.viewportStartOffset + info.viewportEndOffset) / 2
     val item = info.visibleItemsInfo.firstOrNull { it.index == index }