dda.c: clear /all/ lookahead variables when starting.

Catched it! :-)

This was causing occasional step losses when a movement with
zero crossF followed a pair with high crossF.

dda.c

@@ -113,6 +113,7 @@ void dda_create(DDA *dda, TARGET *target) {
   #ifdef LOOKAHEAD
     // Set the start and stop speeds to zero for now = full stops between
     // moves. Also fallback if lookahead calculations fail to finish in time.
+    dda->crossF = 0;
     dda->F_start = 0;
     dda->start_steps = 0;
     dda->F_end = 0;

dda_lookahead.c

@@ -172,7 +172,6 @@ void dda_find_crossing_speed(DDA *prev, DDA *current, uint32_t curr_distance) {
   // Bail out if there's nothing to join (e.g. G1 F1500).
   if ( ! prev || prev->nullmove) {
     prev_distance = curr_distance;
-    current->crossF = 0;
     return;
   }