Is a portable Wok burner safe (indoors)
It can be safe - people use gas burning stoves inside on a regular basis.
The one thing you must do is ensure that there is adequate air-flow in your kitchen so as to prevent the build-up of carbon monoxide. This is as simple as leaving a window partially open when cooking.
Another concern would be how stable one of these is on your bench/work surface. Placing a large pan on top of a small foot-print stove will make it decidedly top heavy, so you should get one that is either low and flat (much like a regular cooking stove in a kitchen; e.g. this one) or one with feet that limit the movement/tipping capacity (e.g. this one). I have no affiliation with either product and make no claims about their ability to cook things, other than that MSR is a reliable outdoors brand. After all, you don't want your cooker to tip over and set your kitchen on fire or cause you to get burned from the hot food/pan and/or from the flame.