Avoid compiler warning about dangling reference

This addresses the following warning:

    warning: possibly dangling reference to a temporary [-Wdangling-reference]
      777 |         const Chunk &chunk = it.next().value();
          |                      ^~~~~

It's a false positive, since even though the temporary was destroyed,
the returned Chunk reference will remain valid...

src/libtiled/tilelayer.cpp

@@ -774,7 +774,8 @@ QVector<QRect> TileLayer::sortedChunksToWrite(QSize chunkSize) const
 
     QHashIterator<QPoint, Chunk> it(mChunks);
     while (it.hasNext()) {
-        const Chunk &chunk = it.next().value();
+        it.next();
+        const Chunk &chunk = it.value();
         if (chunk.isEmpty())
             continue;