box: fix memory leak on error_unpack_unsafe() failure

Memory is leaked in the following scenario:
- MP_ERROR_STACK with 2 errors is passed to error_unpack_unsafe():
  1. A correct MP_MAP with MP_ERROR_* fields;
  2. Something unexpected, e.g. MP_INT;
- This first call to mp_decode_error_one() allocates memory for the first
  error in error_build_xc() -> `new ClientError()`;
- The second call to mp_decode_error_one() returns NULL, and
  error_unpack_unsafe() returns NULL too. Memory from the previous step
  is leaked.

Closes #8921

NO_DOC=bugfix