util: LogCommandLine: argc is an int, so assert that it's positive

util: LogCommandLine: argc is an int, so assert that it's positive

In practice, argc should never be negative, but by convention it's
a signed int, so change the assertion to require it to be positive
rather than merely nonzero to get some help to the static analyzer.

Change-Id: I2d6c00b8ad2f8d573d717f1e2b88a4bb902f125c
Reviewed-on: http://gerrit.openafs.org/7765
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>