QQuickCustomParticle: Check for current OpenGL Context before use

QQuickCustomParticle::buildCustomNodes() assumes there is a valid OpenGL
context, but when there is not it will simply crash.  Instead we check
for a valid current OpenGL context first, and return 0 if it is not
availalbe.  This needed for the Qt Quick 2d Renderer.

Change-Id: I6bfcfc8fa9581bfd27015f719fc527c36492eade
Reviewed-by: aavit <eirik.aavitsland@digia.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>