Fix integer comparison safety issues (6 HIGH priority)

Added parameter expansion with defaults to prevent comparison errors
on potentially empty variables:

- live-attack-monitor-v2.sh: IPSET_CREATE_EXIT, IPTABLES_EXIT
- live-attack-monitor.sh: IPSET_CREATE_EXIT, IPTABLES_EXIT
- malware-scanner.sh: START_EXIT
- email-diagnostics.sh: check_type, account_found

Pattern: Changed "$VAR" to "${VAR:-default}" in integer comparisons
to ensure safe comparisons even if variable is unexpectedly empty.

modules/email/email-diagnostics.sh

@@ -709,7 +709,7 @@ if [ "$greylist" -gt 0 ] && [ "$delivered" -eq 0 ]; then
     echo ""
 fi
 
-if [ "$check_type" != "2" ] && [ $account_found -eq 0 ]; then
+if [ "${check_type:-1}" != "2" ] && [ "${account_found:-0}" -eq 0 ]; then
     echo "Email account not found:"
     echo "  1. Verify the email address is spelled correctly"
     echo "  2. Check if domain DNS points to this server"