handle EOF when checking archive validity

[jsvd]

if the gzip file is truncated the EOF exception must be handled during archive validity check.

To test, create a truncated gzip file:

/tmp ❯ echo "1234567890" | gzip > a.gz
/tmp ❯ dd if=a.gz of=a.truncated.gz bs=1 count=5
5+0 records in
5+0 records out
5 bytes transferred in 0.000263 secs (19011 bytes/sec)

Then start Logstash:

bin/logstash -e "input { file { path => '/tmp/a.truncated.gz' sincedb_path => '/dev/null' mode => read check_archive_validity => true start_position => 'beginning' file_completed_action => log file_completed_log_path => '/tmp/log' } } "

closes #300

[andsel]

LGTM,
however it's not clear why Travis CI goes in timeout

[jsvd]

@logstashmachine bump patch