gse_krb5: Avoid a segfault when we can not read the dedicated keytab file

gse_krb5: Avoid a segfault when we can not read the dedicated keytab file

This improved code simply cleans up the memory as soon as possible,
rather than using memcmp().  Otherwise, we segfault if
krb5_kt_start_seq_get fails, as it can set the fd element in the
handle to -1.

Change-Id: Ib4821ef944a7e12cd8a891ae07dbfc0567c65495
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Pair-programmed-with: Garming Sam <garming@catalyst.net.nz>
Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Nov 27 07:38:02 CET 2014 on sn-devel-104