cls_rgw_client.cc: fix potential null pointer deref

Fix for:

CID 1054853 (#1 of 1): Dereference before null check (REVERSE_INULL)
 check_after_deref: Null-checking is_truncated suggests that it may
 be null, but it has already been dereferenced on all paths leading
 to the check.

Add vim line to file.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>