fix data race

Author: wanglei6

internal/pool/pool.go

@@ -93,8 +93,9 @@ func NewConnPool(opt *Options) *ConnPool {
 		idleConns: make([]*Conn, 0, opt.PoolSize),
 		closedCh:  make(chan struct{}),
 	}
-
+	p.connsMu.Lock()
 	p.checkMinIdleConns()
+	p.connsMu.Unlock()
 
 	if opt.IdleTimeout > 0 && opt.IdleCheckFrequency > 0 {
 		go p.reaper(opt.IdleCheckFrequency)