afscp: Fix check for bare root.cell dirs in dynroot mode

A previous fix (git commit fb1d7491fbe2e90300b23284f213cac2bdcd56ac)
added a check for '!p' in gettoproot(), but p is always a non-NULL pointer
(in part since it's dereferenced just above in the code), so the check
is always false.

Instead, I suspect the original author intended to check for '!*p',
which this patch does.

Change-Id: I1c0a2dd1d582a6b5e82275db18474343efa679a2
Reviewed-on: http://gerrit.openafs.org/9337
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>