rgw: RGWSwiftWebsiteHandler::is_web_dir checks empty subdir_name

checking for empty name avoids later assertion in RGWObjectCtx::set_atomic

Fixes: CVE-2021-3531

Reviewed-by: Casey Bodley <cbodley@redhat.com>
Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 7196a469b4470f3c8628489df9a41ec8b00a5610)

src/rgw/rgw_rest_swift.cc

@@ -2545,6 +2545,9 @@ bool RGWSwiftWebsiteHandler::is_web_dir() const
     return false;
   } else if (subdir_name.back() == '/') {
     subdir_name.pop_back();
+    if (subdir_name.empty()) {
+      return false;
+    }
   }
 
   rgw_obj obj(s->bucket, std::move(subdir_name));