Made plan_buffer_line arguments const, which optimizes better and fixes the RUNOUT_PREVENTION feature.

Marlin/planner.cpp

 // Add a new linear movement to the buffer. steps_x, _y and _z is the absolute position in 
 // mm. Microseconds specify how many microseconds the move should take to perform. To aid acceleration
 // calculation the caller must also provide the physical length of the line in millimeters.
-void plan_buffer_line(float &x, float &y, float &z, float &e, float feed_rate, uint8_t &extruder)
+void plan_buffer_line(const float &x, const float &y, const float &z, const float &e, float feed_rate, const uint8_t &extruder)
 {
   // Calculate the buffer head after we push this byte
   int next_buffer_head = next_block_index(block_buffer_head);

Marlin/planner.h

 
 // Add a new linear movement to the buffer. x, y and z is the signed, absolute target position in 
 // millimaters. Feed rate specifies the speed of the motion.
-void plan_buffer_line(float &x, float &y, float &z, float &e, float feed_rate, uint8_t &extruder);
+void plan_buffer_line(const float &x, const float &y, const float &z, const float &e, float feed_rate, const uint8_t &extruder);
 
 // Set position. Used for G92 instructions.
 void plan_set_position(const float &x, const float &y, const float &z, const float &e);