fixed a nil dereferencing bug in planner_forward_pass_kernel thanks to jv4779

planner.c

@@ -206,7 +206,8 @@ static void planner_forward_pass_kernel(block_t *previous, block_t *current, blo
   // If the previous block is an acceleration block, but it is not long enough to 
   // complete the full speed change within the block, we need to adjust out entry
   // speed accordingly. Remember current->entry_factor equals the exit factor of 
-  // the previous block.
+  // the previous block.¨
+  if(previous) {
     if(previous->entry_factor < current->entry_factor) {
       double max_entry_speed = max_allowable_speed(-settings.acceleration,
         current->nominal_speed*previous->entry_factor, previous->millimeters);
@@ -215,6 +216,7 @@ static void planner_forward_pass_kernel(block_t *previous, block_t *current, blo
         current->entry_factor = max_entry_factor;
       }
     }
+  }
 }
 
 // planner_recalculate() needs to go over the current plan twice. Once in reverse and once forward. This