Fix crash on editing qmake path

Invalid qmake's would lead to crashes

Task-Nr: QTCREATORBUG-5544

Change-Id: Iba3d4004a7d242f0ff5e56556befa82b58e68bbb
Reviewed-on: http://codereview.qt.nokia.com/2277
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>

src/plugins/qtsupport/qtoptionspage.cpp

@@ -629,6 +629,8 @@ void QtOptionsPageWidget::editPath()
     if (qtVersion.isNull())
         return;
     BaseQtVersion *version = QtVersionFactory::createQtVersionFromQMakePath(qtVersion);
+    if (!version)
+        return;
     // Same type? then replace!
     if (current->type() != version->type()) {
         // not the same type, error out