Fixes issue #2: eval script decompression bug

If any of the extensions .zip, .tar.gz, or .tgz does not have at least one matching archive in the LDC assessment directory, then the script will fail due to a quirk of bash that "for f in *.tar.gz; do" will set f to "*.tar.gz" if there are no matching files, instead of skipping the body of the for loop entirely.

We disable this behavior with shopt -s nullglob before using these globs and then restore it when we are done.