Fix Z_HEAD index for COREYZ

Marlin/planner.cpp

@@ -782,7 +782,7 @@ void Planner::check_axes_activity() {
    * Having the real displacement of the head, we can calculate the total movement length and apply the desired speed.
    */
   #if ENABLED(COREXY) || ENABLED(COREXZ) || ENABLED(COREYZ)
-    float delta_mm[6];
+    float delta_mm[7];
     #if ENABLED(COREXY)
       delta_mm[X_HEAD] = dx * steps_to_mm[A_AXIS];
       delta_mm[Y_HEAD] = dy * steps_to_mm[B_AXIS];