uptane iterator: Fix null pointer bug
uptane iterator: Fix null pointer bug

If renewTargetsData fails, cur_targets_ won't be availble and will
cause core dump.

Signed-off-by: Andy Doan <e3579b1e47f273529f0f929453e939a68ede9fd1@foundries.io>

advancedtelematic/aktualizr,advancedtelematic/aktualizr,advancedtelematic/sota_client_cpp,advancedtelematic/sota_client_cpp,advancedtelematic/aktualizr,advancedtelematic/aktualizr