Fixed deprecated warning on log

Added check of array to be a minimum of 2

PHP Warning:  Undefined array key -1 in /usr/local/ispconfig/server/lib/classes/cron.d/100-monitor_clamav_log.inc.php on line 111

PHP Deprecated:  strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated in /usr/local/ispconfig/server/lib/classes/cron.d/100-monitor_clamav_log.inc.php on line 111

Signed-off-by: Nephiaust <29741794+Nephiaust@users.noreply.github.com>