RGWPutCORS_ObjStore_S3::get_params: check data before dereference

CID 1063697 (#1 of 1): Explicit null dereferenced (FORWARD_NULL)
 5. var_deref_model: Passing null pointer "data" to function
 "RGWXMLParser::parse(char const *, int, int)", which dereferences it.

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