Corrected e-length calculation in planner

Marlin/planner.cpp

   delta_mm[Z_AXIS] = (target[Z_AXIS]-position[Z_AXIS])/axis_steps_per_unit[Z_AXIS];
   delta_mm[E_AXIS] = (target[E_AXIS]-position[E_AXIS])/axis_steps_per_unit[E_AXIS];
   if ( block->steps_x == 0 && block->steps_y == 0 && block->steps_z == 0 ) {
-    block->millimeters = delta_mm[E_AXIS];
+    block->millimeters = abs(delta_mm[E_AXIS]);
   } else {
     block->millimeters = sqrt(square(delta_mm[X_AXIS]) + square(delta_mm[Y_AXIS]) + square(delta_mm[Z_AXIS]));
   }