statstool: ensure all unused time fields are zero

Ensure that all the struct tm fields are zero else we get occasional
garbage from the stack into the time, which is bad.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
statstool: fix stupid tm_year 1900 offset bug

I was stupidly not passing tm_year to mktime in terms of years since
1900, which causes overflow on 32 bit systems making the mktime return
-1 which in turn causes crazy times and breaks the stats calculations.

To fix this ensure tm_year has 1900 subtracted from it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>