Fallback renderloop will crash if QOpenGLContext::create fails.

Accidentally I have found out that Qt will crash when gl context will not be created. See following

https://qt.gitorious.org/qt/qtdeclarative/source/21a08ccde3c7c2dc69e06b714549585bbf39b5c3:src/quick/scenegraph/qsgrenderloop.cpp#L254

current = gl->makeCurrent(window);
will crash.
shouldn't be return after gl=0 or something like this?

Sorry but I don't have time to investigating it more.