userdb: cleanup use of ERRNO_IS_PRIVILEGE()

userdb: cleanup use of ERRNO_IS_PRIVILEGE()

Given that ERRNO_IS_PRIVILEGE() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the argument passed to ERRNO_IS_PRIVILEGE() is the value
returned by json_parse_file() which is not expected to return any positive
values, but let's be consistent anyway and move the ERRNO_IS_PRIVILEGE()
invocation to the branch where the return value is known to be negative.