Fix spin loop reading from KDC TCP socket

In the k5_sendto code for reading from a TCP socket, detect
end-of-stream when reading the length.  Otherwise we can get stuck in
an infinite loop of poll() and read().

[ghudson@mit.edu: commit message]

ticket: 7508
target_version: 1.11.4
tags: pullup
KDC null deref due to referrals [CVE-2013-1417]

An authenticated remote client can cause a KDC to crash by making a
valid TGS-REQ to a KDC serving a realm with a single-component name.
The process_tgs_req() function dereferences a null pointer because an
unusual failure condition causes a helper function to return success.

While attempting to provide cross-realm referrals for host-based
service principals, the find_referral_tgs() function could return a
TGS principal for an zero-length realm name (indicating that the
hostname in the service principal has no known realm associated with
it).

Subsequently, the find_alternate_tgs() function would attempt to
construct a path to this empty-string realm, and return success along
with a null pointer in its output parameter.  This happens because
krb5_walk_realm_tree() returns a list of length one when it attempt to
construct a transit path between a single-component realm and the
empty-string realm.  This list causes a loop in find_alternate_tgs()
to iterate over zero elements, resulting in the unexpected output of a
null pointer, which process_tgs_req() proceeds to dereference because
there is no error condition.

Add an error condition to find_referral_tgs() when
krb5_get_host_realm() returns an empty realm name.  Also add an error
condition to find_alternate_tgs() to handle the length-one output from
krb5_walk_realm_tree().

The vulnerable configuration is not likely to arise in practice.
(Realm names that have a single component are likely to be test
realms.)  Releases prior to krb5-1.11 are not vulnerable.

Thanks to Sol Jerome for reporting this problem.

CVSSv2: AV:N/AC:M/Au:S/C:N/I:N/A:P/E:H/RL:O/RC:C