s4:torture: Use strlcpy() in gen_name()

s4:torture: Use strlcpy() in gen_name()

../source4/torture/basic/mangle_test.c: In function ‘gen_name’:
../source4/torture/basic/mangle_test.c:148:3: error: ‘strncpy’ output
    truncated before terminating nul copying 5 bytes from a string of the
    same length [-Werror=stringop-truncation]
   strncpy(p, "ABCDE", 5);
   ^~~~~~~~~~~~~~~~~~~~~~

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13437

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>