That's a very perceptive six year-old you have there, Olle, and in all fairness I think he should get credit for the sharpness of his observation.
I agree with Harrock that the branch would have likely been from something like an acacia tree, but I do see it as a weakness that Hergé didn't actually set up its production, by either showing the jagged remains of a tree amongst the rocks, or by showing dead branches strewn about the ground as Müller and then Tintin move around.
I think we are actually watching the insertion of what I have heard referred to in terms of film as a "utility prop": it may not have existed before, it could disappear immediately, but some item needed by the person on screen will manifest itself for as long as it is required, and no longer.
So, in the movie Jurassic Park
, the character Nedry has to wipe his hands, and manages to do it (twice) on a napkin that just magically appears, then disappears, without him having to pick it up or put it down.
There's a similar class of things which, having been established as being there, then vanish if they will impede the action.
The text-book example of that is in Lethal Weapon
, when Riggs attaches himself with handcuffs to the man contemplating suicide, makes a big
thing out of how they now cannot be separated whatever happens
because of the cuffs, and then forces the man into jumping off the building with him - at which point there are no hand-cuffs and they are not falling together
See luinivierge2010's post here
, for another example of a randomly inserted prop, and the explanation why...