Add a ZERO macro to clear arrays

Marlin/Marlin_main.cpp

       if (DEBUGGING(LEVELING)) DEBUG_POS(">>> home_delta", current_position);
     #endif
     // Init the current position of all carriages to 0,0,0
-    memset(current_position, 0, sizeof(current_position));
+    ZERO(current_position);
     sync_plan_position();
 
     // Move all carriages together linearly until an endstop is hit.

Marlin/SdBaseFile.cpp

       index = 0;
     }
     // initialize as empty file
-    memset(p, 0, sizeof(dir_t));
+    ZERO(p);
     memcpy(p->name, dname, 11);
 
     // set timestamps

Marlin/cardreader.cpp

   sdpos = 0;
   workDirDepth = 0;
   file_subcall_ctr = 0;
-  memset(workDirParents, 0, sizeof(workDirParents));
+  ZERO(workDirParents);
 
   autostart_stilltocheck = true; //the SD start is delayed, because otherwise the serial cannot answer fast enough to make contact with the host software.
   autostart_index = 0;

Marlin/macros.h

 #define NUMERIC(a) ((a) >= '0' && '9' >= (a))
 #define NUMERIC_SIGNED(a) (NUMERIC(a) || (a) == '-')
 #define COUNT(a) (sizeof(a)/sizeof(*a))
+#define ZERO(a) memset(a,0,sizeof(a))
 
 // Macros for initializing arrays
 #define ARRAY_6(v1, v2, v3, v4, v5, v6, args...) { v1, v2, v3, v4, v5, v6 }

Marlin/mesh_bed_leveling.cpp

   void mesh_bed_leveling::reset() {
     status = MBL_STATUS_NONE;
     z_offset = 0;
-    memset(z_values, 0, sizeof(z_values));
+    ZERO(z_values);
   }
 
 #endif  // MESH_BED_LEVELING

Marlin/planner.cpp

 
 void Planner::init() {
   block_buffer_head = block_buffer_tail = 0;
-  memset(position, 0, sizeof(position));
-  memset(previous_speed, 0, sizeof(previous_speed));
+  ZERO(position);
+  ZERO(previous_speed);
   previous_nominal_speed = 0.0;
   #if ABL_PLANAR
     bed_level_matrix.set_to_identity();
   stepper.set_position(na, nb, nc, ne);
   previous_nominal_speed = 0.0; // Resets planner junction speeds. Assumes start from rest.
 
-  memset(previous_speed, 0, sizeof(previous_speed));
+  ZERO(previous_speed);
 }
 
 void Planner::set_position_mm_kinematic(const float position[NUM_AXIS]) {