China's army runs on oil. It will be in short supply and very expensive. Also, China will soon be in the throes of dealing with too many huge internal issues to invade Russia. Drought is already wracking their cropland, and the've become a food importer for the first time ever. That situation will only increase, causing internal strife. And if they do manage to invade? What, Siberia? Without oil and gas for heat, it's almost impossible to live there.

Siberia is where most of Russia's oil is.
Having internal troubles often means that a war is sought to divert attention.