rgw: initialize variable before call

Need to initialize the truncated variable, as we sometimes ignore error
response (e.g., with ENOENT), and in such cases we can't expect it to be
set.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1191 from ceph/wip-rgw-vg

rgw: initialize variable before call

Revewed-by: Sage Weil <sage@inktank.com>