x/text: UTF-16 decoder behaves incorrectly on single-byte input

When using UTF-16 decoder with BOM to decode a single-byte string, the decoder incorrectly returns `ErrShortSrc`. This code can be used to reproduce this issue:

```go
package main

import (
	"fmt"
	"golang.org/x/text/encoding/unicode"
)

func main() {
	res, err := unicode.UTF16(unicode.BigEndian, unicode.UseBOM).NewDecoder().String(" ")
	fmt.Println(res, err)
}
```