Fix GSS krb5 initial sequence number gap handling

Fix GSS krb5 initial sequence number gap handling

Since #2040, the dummy queue element inserted by g_order_init no
longer compares less than the initial sequence number, so we fail when
the first few sequence numbers are received out of order.  Properly
detect when a sequence number fits between the dummy element and the
first real queue element.

[ghudson@mit.edu: rewrote commit message]

(cherry picked from commit 13a9cb721194c8aa4ccf6ed6ef23e3ac8dd24037)

ticket: 7872
version_fixed: 1.12.2
status: resolved