TestCase does not ensure capture buffers are not null before checking them

[eljonny]

Encountered this while trying to increase code coverage for #10.

[eljonny]

Encountered again trying to get sufficient patch coverage for #27 which is for resolving #8.
My bad for accidentally not selecting the right check function but it should just return false and write a warning (or maybe throw an exception, idk yet), not cause a SEGFAULT/Access Violation.