client: fix sending keepalives (bluenviron/mediamtx#1812)

this fixes a regression introduced with #277

client.go

@@ -943,7 +943,7 @@ func (c *Client) do(req *base.Request, skipResponse bool, allowFrames bool) (*ba
 		c.session = sx.Session
 
 		if sx.Timeout != nil && *sx.Timeout > 0 {
-			c.keepalivePeriod = time.Duration(((*sx.Timeout)*10)/8) * time.Second
+			c.keepalivePeriod = time.Duration(*sx.Timeout) * time.Second * 8 / 10
 		}
 	}