Fix unsigned integer overflow in rrdtool first

Fix unsigned integer overflow in rrdtool first

This fixes a signed/unsigned conversion bug in the calculation of
"then". Background info:
pdp_cnt and pdp_step are both unsigned long, whereas timer is signed.
When multiplying signed and unsigned integers (same size), a signed is
implicitly typecast to unsigned.

- A similar fix has already been applied to rrd_dump.c
  in commit e193975
- Resolves #1140