Fix bug reading data frames

There was a bug keeping track of the last read frame of data blocks. There was also an issue trying to access data that could have been freed.

mentioned in merge request !3 (merged)

approved this merge request

are you okay if I approve @acmnpv?

Sure, go ahead!

approved this merge request

started a merge train

@magnus.lundborg I would merge this in now. @hugomacdermott rebasing your new tests on this and activating the previously failing part will be great to check that this worked as intended

resolved all threads

merged

mentioned in issue #4 (closed)

mentioned in issue #7 (closed)