auth/authcon.c: fix likely mistake

auth/authcon.c: fix likely mistake

It seems likely the original developer intended to check
*scIndex==RX_SECIDX_NULL rather than scIndex==RX_SECIDX_NULL.

Change-Id: I8a33d3cb177a9212917613581957073392b08314
Reviewed-on: http://gerrit.openafs.org/8881
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>