rgw: fix double slash (//) killing the gateway
When a bucket is inialized as a static website, a curl request on the bucket with double slash kills the gateway.
The problem is on the URL handling of the subdirectory, which tries to remove the last slash of any URL, so when only / is given as a sub-directory, this results to an empty string.
Fixes: https://tracker.ceph.com/issues/41225
Signed-off-by: Theofilos Mouratidis <t.mour@cern.ch>
(cherry picked from commit
626330c504b1af8393986bd0553786c3031422cd)