Fix planner wrong trap generation

If the planner `entry_rate` or `final_rate` are larger thanthe  `block->nominal_rate` then the trapezoid entry ramp continuously accelerates. Only happens if feed rate is less than MAXIMAL_STEP_RATE.

Marlin/src/module/planner.cpp

@@ -796,6 +796,10 @@ void Planner::calculate_trapezoid_for_block(block_t * const block, const_float_t
 
   uint32_t final_rate = block->nominal_rate;                          // (steps per second)
   if (exit_factor < 1.0f) final_rate *= exit_factor;
+  // Limit minimal step rate (Otherwise the timer will overflow.)
+  NOLESS(initial_rate, uint32_t(MINIMAL_STEP_RATE));
+  NOLESS(final_rate, uint32_t(MINIMAL_STEP_RATE));
+  NOLESS(block->nominal_rate, (uint32_t)MINIMAL_STEP_RATE);
 
   #if ANY(S_CURVE_ACCELERATION, LIN_ADVANCE)
     // If we have some plateau time, the cruise rate will be the nominal rate