fix: avoid out-of-range index for serverSaslCreds in saslBindTokenExchange

[netliomax25-code]

1. saslBindTokenExchange reads serverSaslCreds at protocolOp.Children[3] for a saslBindInProgress (result code 14) response, but the length check ahead of it only required three children.
2. serverSaslCreds is the optional [7] field that follows the three LDAPResult components, so a server that returns code 14 with only those components makes the index out of range and panics the bind goroutine mid GSSAPI handshake.

Require at least four children before reading the creds child, so a short response falls through to GetLDAPError instead of indexing past the slice. Added a regression test that drives the exchange with a three-child in-progress response.

[netliomax25-code]

pushed a fix for the lint failure. errcheck flagged the deferred Close calls in the new test, and the branch also needed a rebase onto master so the linter stopped attributing pre-existing lines to this diff. checks are green now.