Don't crash when render node doesn't have a batchroot

Change-Id: Ib8f70414eb477b4ec45097c8784a5fe1375cf012
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

Conflicts:
	src/quick/scenegraph/coreapi/qsgbatchrenderer.cpp
Merge pull request #8 from sletta/rendernode-crash

[qtdeclarative] Don't crash when render node doesn't have a batchroot