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>
(cherry picked from commit 9ecf3467a3efc071954c4f69a23b51e677742c1f)