log: catch invalid level option number

log: catch invalid level option number

The parsing check for invalid log level was not trying to catch
irrelevant numeric values.
A log level 0 becomes a failure in parsing so it can be caught early.
A log level higher than the max (8) is accepted with a warning message.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: David Marchand <david.marchand@redhat.com>