[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
Merge pull request #2739 from KFilipek/XWALK-3206

[Tizen] Fix memory leaks found in xwalk_tizen_user.cc