Avoid 'salvageserver -client -showlog' segfault

Avoid 'salvageserver -client -showlog' segfault

Running salvageserver with the -client and -showlog options will
currently segfault, since -client does not open logFile, and -showlog
will attempt to rewind logFile on exit.

Fix this by not allowing -client and -showlog together (since it won't
work anyway, as -showlog tries to read SalvageLog), and by making
showlog() check logFile for NULL-ness.

Change-Id: I30f43ef1696f1e7e8873b9e1cc3867b4e06d1980
Reviewed-on: http://gerrit.openafs.org/734
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>