heater.c:initialize heater_i to 0.
dda_lookahead.c: Eliminate debug crossF uninitialized variable compile warning.

Fix:
  dda_lookahead.c:327:17: warning: 'crossF' may be used uninitialized in this function [-Wmaybe-uninitialized]
       sersendf_P(PSTR("Initial crossing speed: %lu\n"), crossF);
                 ^