Fix potential deadlock in merge.go

[LEAVING-7]

rosedb/merge.go

Lines 88 to 92 in d392613

	// rotate the write-ahead log, create a new active segment file.
	// so all the older segment files will be merged.
	if err := db.dataFiles.OpenNewActiveSegment(); err != nil {
	return err
	}

It seems that the lock isn't released when the OpenNewActiveSegment function returns an error

[roseduan]

thanks