[Tizen] Fix memory leaks found in xwalk_tizen_user.cc

Memory leaks appear when xwalk_backend runs correctly.
This patch adds freeing buffer after usage.

BUG=XWALK-3206
It makes more sense to drop the C-ism and bring `buffer`'s declaration closer to the usage. And there's no need to check for `!buffer` below:

```c++
scoped_ptr<char[]> buffer(new char[len]);
err = getgrgid_r(...);
```