Fix potential crash when clicking in a text edit

Since the attributes() call might resize the layout and therefore
delete the previous data, the logClusters pointer might be dangling
at this point. We need to reget it to make sure it's valid.

Task-number: QTBUG-20310
Reviewed-by: Jiang Jiang
(cherry picked from commit c0772f44fb1d53608ff629fc622103698c6d0ee5)

Change-Id: I20a29d0c529764eb5d41cb3383c22b6cad8de255
Reviewed-on: http://codereview.qt.nokia.com/1598
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>

src/gui/text/qtextengine.cpp

@@ -2872,6 +2872,7 @@ int QTextEngine::positionInLigature(const QScriptItem *si, int end,
     }
 
     const HB_CharAttributes *attrs = attributes();
+    logClusters = this->logClusters(si);
     clusterLength = getClusterLength(logClusters, attrs, 0, end, glyph_pos, &clusterStart);
 
     if (clusterLength) {