pam_krb5: Clarify a loop condition.

The closing parenthesis was in the wrong location, so instead of assigning the return value to krbret and then comparing it to zero, we were assigning the result of the comparison to krbret and then comparing that to zero. This has no practical significance since the value is not used after the loop terminates.

PR: 229719
Reviewed by: cy
Differential Revision: https://reviews.freebsd.org/D41299