rgw: check for tagging element in POST Obj requests

Check for null element when reading the tagging field from POST obj XML

Fixes: https://tracker.ceph.com/issues/44967
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>

src/rgw/rgw_rest_s3.cc

@@ -1786,6 +1786,9 @@ int RGWPostObj_ObjStore_S3::get_tags()
     RGWObjTagging_S3 *tagging;
 
     tagging = static_cast<RGWObjTagging_S3 *>(parser.find_first("Tagging"));
+    if (!tagging) {
+      return -ERR_MALFORMED_XML;
+    }
     obj_tags_s3 = static_cast<RGWObjTagSet_S3 *>(tagging->find_first("TagSet"));
     if(!obj_tags_s3){
       return -ERR_MALFORMED_XML;