stop leaking a temp dir if starting Chrome fails
stop leaking a temp dir if starting Chrome fails

This was a minor bug in ExecAllocator.Allocate; see the added comment.
Keeps 'go test' from leaving a couple dozen empty directories in /tmp.

knq/chromedp,knq/chromedp,knq/chromedp,chromedp/chromedp,chromedp/chromedp,chromedp/chromedp