Corrected distance calculation. (thanks jv4779)

Marlin/planner.cpp

   delta_mm[Y_AXIS] = (target[Y_AXIS]-position[Y_AXIS])/axis_steps_per_unit[Y_AXIS];
   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];
-  block->millimeters = sqrt(square(delta_mm[X_AXIS]) + square(delta_mm[Y_AXIS]) +
-                            square(delta_mm[Z_AXIS]) + square(delta_mm[E_AXIS]));
+  if ( block->steps_x == 0 && block->steps_y == 0 && block->steps_z == 0 ) {
+    block->millimeters = delta_mm[E_AXIS];
+  } else {
+    block->millimeters = sqrt(square(delta_mm[X_AXIS]) + square(delta_mm[Y_AXIS]) + square(delta_mm[Z_AXIS]));
+  }
   float inverse_millimeters = 1.0/block->millimeters;  // Inverse millimeters to remove multiple divides 
   
   // Calculate speed in mm/second for each axis. No divide by zero due to previous checks.
   block->nominal_speed = block->millimeters * inverse_second; // (mm/sec) Always > 0
   block->nominal_rate = ceil(block->step_event_count * inverse_second); // (step/sec) Always > 0
 
-  
- 
-
   if (block->steps_e == 0) {
         if(feed_rate<mintravelfeedrate) feed_rate=mintravelfeedrate;
   }
     	if(feed_rate<minimumfeedrate) feed_rate=minimumfeedrate;
   } 
 
-
   // slow down when de buffer starts to empty, rather than wait at the corner for a buffer refill
   int moves_queued=(block_buffer_head-block_buffer_tail + BLOCK_BUFFER_SIZE) & (BLOCK_BUFFER_SIZE - 1);
 #ifdef SLOWDOWN