Wrong check for errno values in tests of ft_write

[JonathanDUFOUR]

The last test of the function ft_write checks that the function returns a correct error value (-1) and that it sets correctly the value of errno depending on the error that occurred. In that test, NULL is passed as the second argument, and the currently expected errno value is EINVAL, but this is wrong. Indeed, the correct expected errno value is EFAULT.