rgw: fix multi delete

Fixes: #5931
Backport: bobtail, cuttlefish

Fix a bad check, where we compare the wrong field. Instead of
comparing the ret code to 0, we compare the string value to 0
which generates implicit casting, hence the crash.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit f9f1c48ad799da2b4be0077bf9d61ae116da33d7)

Conflicts:
	src/rgw/rgw_rest_s3.cc