We generally hear the words “careful what you wish for” enough times in life that you’d think, if you ever were to be granted your heart’s desire by supernatural means, you’d at least be really particular about the wording. But an impetuous request to an invisible genie-like entity has ruinous consequences for multiple people in “ Obsession ,” a nifty, unnerving little horror film from internet comedian and viral-video auteur Curry Barker that begins like the premise of a joke — and ends with a far sicker punchline than you might expect. Like the novelty gift that causes all the trouble, “Obsession” initially seems simplistic, and even a bit silly, in its rehash of the age-old monkey’s paw trope. Like the consequences of that ill-considered wish, however, it proves eerily hard to shake.…