Modify the compile error about ftruncate()

Summary:
Change to store the return value from ftruncate().
The reason is that ftruncate() has "warn_unused_result" attribute in some environment.

Signed-off-by: Yumikiyo Osanai <yumios.art@gmail.com>
Merge pull request #94 from yumiOS/modify_ftruncate_warning

Modify the compile error about ftruncate()