[NaCl SDK] nacl_io: fix for opening files with O_TRUNC in memfs

Add corresponding test.

R=binji@chromium.org

Review URL: https://codereview.chromium.org/60803005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233934 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233934