Assertion in "boost::icl::intersects" for empty interval

The following example gives an assertion with boost 1.66.0 under Windows:
```
typedef boost::icl::discrete_interval interval_t;
typedef boost::icl::interval_set interval_set_t;

interval_t int1 = interval_t::right_open(2, 7);
interval_t int2 = interval_t::right_open(3, 3);
interval_set_t intset;
intset.insert(int1);

// OK, returns "false":
bool b1 = intersects(int1, int2);

// Assertion in "boost::icl::non_empty::exclusive_less",
// because interval "right" is empty:
bool b2 = intersects(intset, int2);
```
Expected behavior would be that the second call to `boost::icl::intersects` also returns "false".