s4:torture: Initialize tm struct

s4:torture: Initialize tm struct

The samba3.blackbox.shadow_copy_torture tests call to strptime passing
an uninitalized tm structure as an argument, but the strptime function
does not write the tm.tm_isdst field.

These tm structures are passed later as the mktime argument, which
produces different values depending on whether the arbitrary value
of the tm.tm_isdst field is lower or equal to zero or greather than
zero.

Signed-off-by: Samuel Cabrero <scabrero@suse.de>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>