You still missed the point, sorry :)
What I was saying is that your premise - "if he crashes the economy his country is no longer a threat to the world" - is incorrect. For one thing, it's always tempting to "fix" the crashed economy by going to war. But even if it doesn't actually fix it, it can be that last "okay, if we're going down, you're going down with us" sort of gesture. Yes, a country with a ruined economy won't be able to wage a protracted war, but it doesn't need to do so to make others hurt, and the bombs and the missiles won't magically disappear. Nor will the manpower - and said manpower is only going to be more desperate and therefore (with the right coaching) more angry.
Now, as to why I believe that such a war would work to bolster inner popularity. The trick, of course, is to present it in such a manner that the war is declared on you. Russian TV has already been quite successful at spinning things that way about Ukraine - a recent poll showed that 94% of Russians get their news primarily from TV channels (all of which are now state-run or indirectly state-controlled), and 75% believe that its coverage is truthful and objective. Only 25% believe that "propaganda" is an apt description for what they're seeing.
So, really, all Putin needs to do to escalate to war is to keep provoking the West, and then blowing up any responses as something big. And heck, there are tried and proven methods to get a decent casus belli when the time comes - see Mainila incident for an example. After all the crazy conspiracy theories that are eagerly accepted for granted in Russia (by the population, not by politicians!) just to be able to preserve the "we are the good guys" mentality... something like that would be swallowed very easily.
And yes, the "patriotic" fervor in Russia today is such that, with the right sugar-coating, the population will happily swallow the war pill. If they are explained that all economic woes are due to Western shenanigans (and the occasional spy/saboteur - for the sake of some public circus).
Hell, they are already clamoring for war, seemingly more so than the government itself. Did you see #PutinVvediVoiska ("Putin, move the armed forces in!" [to Ukraine]) Twitter hashtag? It's only growing in popularity as more sanctions come in. Then there's another thing where people are mocking the sanctions themselves - that is also going pretty strong.