Statically proving the variables get initialized wouldn't change the performance except by making sure you check the return value of sscanf, or turning refusal to check into a couple register wipes. Either way, that's a negligible increase to a hefty function call. It wouldn't require default initializing variables in all circumstances.

When I think of the "no runtime cost" mentality of C/C++ I don't think that normally extends to ignoring errors in I/O functions.