Fix back() of empty deque in context_mm_black test

Fix back() of empty deque in context_mm_black test

The `testPushPop()` test case does a pop out of scope at the end that
lead to UB in `ContextManager::pop()` because it did a `deque::back()`
on an empty deque without checking. This commit adds an assertion in the
`ContextManager` and checks that the test case triggers the assertion.