sim_core.c: fix implimentation of inc/dec to match specification

the avr instruction manual states that overflow be set _before_ the operation takes place, however the core tests based on the result.  values checked adjusted accordingly.

bug find credit goes to: Shay Green <gblargg@gmail.com>
Merge pull request #27 from bsekisser/sim-core-inc-dec-bugfix

sim_core.c: fix implimentation of inc/dec to match specification