SG renderer accesses null pointer when shader can't be linked

See Renderer::renderMergedBatch() + about 50 lines:

ShaderManager::Shader *sms = m_useDepthBuffer ? m_shaderManager->prepareMaterial(material) : m_shaderManager->prepareMaterialNoRewrite(material);
QSGMaterialShader *program = sms->program;

The sms variable can be a null pointer here. Avoiding the null pointer access and printing an error message would be helpful.

This might happen on other platforms where the material doesn't link, but the bug is opened because it can happen quite commonly on WinRT if the D3D compiler is left out of the package or the compiler service isn't running.

It's worth noting that there are already debug messages printed by QOpenGLShader and d3dcompiler_qt saying that something went wrong. However, it might be worth going the extra mile and avoiding the crash. Or, qFatal could be used so that a message box could be shown in release builds instead of simply crashing the application.