Complete double_value so that it doubles, in place, the int its pointer parameter points to. main is
already written to read n, call double_value(&n), and print the result – you’re only filling in the function
body.
This is the smallest possible out-parameter: no return value at all, just a pointer the function writes through.
If you find yourself trying to return something from double_value, stop – the whole point of this exercise is
modifying the caller’s variable through its address instead.