normalize:fix a crash bug when object is not normalizer

I fixed the yoku0825 reports bug.

refs [[groonga-dev,02409]](http://sourceforge.jp/projects/groonga/lists/archive/dev/2014-June/002411.html)

I didn't create the test because of the error case.

The following is the error test.
```
> normalize TokenBigram "aBc"
[[-22,1401957669.16034,0.000719547271728516,"unknown normalizer: <TokenBigram>",[["proc_normalize","proc.c",3015]]],""
> normalize LongText "aBc"
[[-22,1401957690.85734,0.000305891036987305,"unknown normalizer: <LongText>",[["proc_normalize","proc.c",3015]]],""]
> normalize select "aBc"
[[-22,1401957710.69732,0.000345706939697266,"unknown normalizer: <select>",[["proc_normalize","proc.c",3015]]],""]

> normalize NormalizerAuto "aBc"
[[0,1401957736.58533,9.25064086914062e-05],{"normalized":"abc","types":[],"checks":[]}]