Remove Q_ASSERT from qitemmodel autotest

If populateTestData() is passed an unknown type of model, report a
meaningful fatal error rather than failing silently in a release build
and aborting with an uninformative error message in a debug build.

Change-Id: I7ee8d1262bbced304f9f24c2ec9b40a38704d982
Task-number: QTBUG-17582
Reviewed-by: Rohan McGovern
(cherry picked from commit 6b2e89843a573cd7c143bca17cec8e69209d2fbd)

tests/auto/qitemmodel/modelstotest.cpp

@@ -372,7 +372,7 @@ QModelIndex ModelsToTest::populateTestArea(QAbstractItemModel *model)
         return QModelIndex();
     }
 
-    Q_ASSERT(false);
+    qFatal("%s: unknown type of model", Q_FUNC_INFO);
     return QModelIndex();
 }