Reopen log file only once after SIGHUP

Commit b6fa9a44 added a re-openable log handler. The log file is
reopened when a daemon is sent a HUP signal. Due to a bug in the code,
fixed by this patch, the log file would be reopened for every single log
message thereafter.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>