Fix de after PREVENT_DANGEROUS_EXTRUDE

- Set `de` to 0 when `position[E] = target[E]`
- Address regression mention in #1846

Marlin/planner.cpp

     if (de) {
       if (degHotend(active_extruder) < extrude_min_temp) {
         position[E_AXIS] = target[E_AXIS]; //behave as if the move really took place, but ignore E part
+        de = 0; // no difference
         SERIAL_ECHO_START;
         SERIAL_ECHOLNPGM(MSG_ERR_COLD_EXTRUDE_STOP);
       }
       #ifdef PREVENT_LENGTHY_EXTRUDE
         if (labs(de) > axis_steps_per_unit[E_AXIS] * EXTRUDE_MAXLENGTH) {
           position[E_AXIS] = target[E_AXIS]; // Behave as if the move really took place, but ignore E part
+          de = 0; // no difference
           SERIAL_ECHO_START;
           SERIAL_ECHOLNPGM(MSG_ERR_LONG_EXTRUDE_STOP);
         }