fix crash when dropping a file into an empty QTextEdit

Since commit be0bfe09ee we return -1 in
QTextEngine::findItem if the item cannot be found.
Calling code must be prepared for this case.
Otherwise we run into an out-of-bounds vector access later on.

Task-number: QTBUG-42103
Change-Id: Ie338cd1fc3cf0ce1e8edfa59dead635669020a33
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

src/gui/text/qtextlayout.cpp

@@ -2559,6 +2559,10 @@ qreal QTextLine::cursorToX(int *cursorPos, Edge edge) const
     }
     else
         itm = eng->findItem(pos);
+    if (itm < 0) {
+        *cursorPos = 0;
+        return x.toReal();
+    }
     eng->shapeLine(line);
 
     const QScriptItem *si = &eng->layoutData->items[itm];