fix interpolateParams

Author: methane

connection.go

@@ -207,7 +207,10 @@ func (mc *mysqlConn) interpolateParams(query string, args []driver.Value) (strin
 	if err != nil {
 		// can not take the buffer. Something must be wrong with the connection
 		mc.cleanup()
-		return "", err
+		// interpolateParams would be called before sending any query.
+		// So its safe to retry.
+		mc.log(err)
+		return "", driver.ErrBadConn
 	}
 	buf = buf[:0]
 	argPos := 0