*Steps to reproduce:*
1. Configure MSSQL plugin to connect to a SQL Server that is unreachable
2. Let the agent run for an extended period (days/weeks)
3. Observe memory usage growing continuously

*Result:*
Memory usage grows unbounded. In our case, ~1.3GB after a month of failed connections.

*Root cause*:
In `plugin/dbconn/dbconn_item.go`, the `getDbConn()` function calls `sql.Open()` 
which allocates a *sql.DB object. When `PingContext()` fails, the function returns 
without calling `db.Close()`, leaking the connection pool resources.

*Current code (line ~129):*

```go
err = db.PingContext(ctx)
if err != nil {
    return nil, errs.Wrap(err, "failed to ping")
} 

Fix:

err = db.PingContext(ctx)
if err != nil {
    db.Close() // Close to prevent memory leak
    return nil, errs.Wrap(err, "failed to ping")
} 

Environment:

• Windows Server
• Zabbix Agent 2 with MSSQL plugin
• SQL Server unreachable for extended period