QTextEngine: don't pass empty data to bidiReorder

The algorithm in QTextEngine::bidiReorder() doesn't like an empty level
and visual order list. Found by GCC 12's slightly misleading warning:

qtextengine.cpp:3838:29: error: ‘levels’ may be used uninitialized [-Werror=maybe-uninitialized]
qtextengine.cpp:1174:6: note: by argument 2 of type ‘const quint8*’ {aka ‘const unsigned char*’} to ‘static void QTextEngine::bidiReorder(int, const quint8*, int*)’ declared here
qtextengine.cpp:3835:28: note: ‘levels’ declared here
 3835 |     QVarLengthArray<uchar> levels(nItems);
      |                            ^~~~~~

Change-Id: If05aeeb7176e4f13af9afffd16e8490f131f160e
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
(cherry picked from commit 54eae2d610bee85a40050d76fc0b01cf6a181f6e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index 9e294d083e..d32b3d7ecb 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -3837,10 +3837,12 @@ QTextLineItemIterator::QTextLineItemIterator(QTextEngine *_eng, int _lineNum, co
 
     x += eng->alignLine(line);
 
-    QVarLengthArray<uchar> levels(nItems);
-    for (int i = 0; i < nItems; ++i)
-        levels[i] = eng->layoutData->items.at(i + firstItem).analysis.bidiLevel;
-    QTextEngine::bidiReorder(nItems, levels.data(), visualOrder.data());
+    if (nItems > 0) {
+        QVarLengthArray<uchar> levels(nItems);
+        for (int i = 0; i < nItems; ++i)
+            levels[i] = eng->layoutData->items.at(i + firstItem).analysis.bidiLevel;
+        QTextEngine::bidiReorder(nItems, levels.data(), visualOrder.data());
+    }
 
     eng->shapeLine(line);
 }