Workaround for "NOT NULL constraint failed" crash (#1)
    
    This commit just makes parse-access-log.py filter out any match item
    that has missing (i.e. NULL) field values. This is quite rare, but it
    happened once in the Fedora mirror logs from September 16, resulting in
    this crash.
    
    Technically, these are still valid `countme` requests, so a future
    update should probably remove that constraint, and then re-parse the
    logs for September 16 (and any other logs with `countme` items that are
    missing `repo=XXX` or `arch=XXX`). But as of now that's 2 hits out of
    14.5 million in the logs, so we're not missing much by ignoring
    malformed requests like these.