Nested THEN/AND_THEN statements in a loop trigger assert.

The following code, and variations of it, breaks for me using Catch v1.3.3 on Visual Studio 2015:

```
for (int i = 0; i < 10; ++i) {
WHEN("Foo") {
THEN("Bar") {
REQUIRE(true);
}
AND_THEN("Baz") {
REQUIRE(true);
}
}
}
```

This is the failing assert in line 5450:
`assert(m_parent);`