sparse: netsniff-ng, trafgen: fix remaining time(0) warnings

Those are the last occurences of warnings like:

 netsniff-ng.c:697:48: warning: Using plain integer as NULL pointer
 netsniff-ng.c:726:48: warning: Using plain integer as NULL pointer
 ...

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>