BUG: Make sure that sas7bdat parsers memory is initialized to 0 (#21616)
Memory for numbers in sas7bdat-parsing was not initialized properly to 0.
For sas7bdat files with numbers smaller than 8 bytes this made the
least significant part of the numbers essentially random.
Fix it by initializing memory correctly.

- [X] closes #21616
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry
