Fix bilinear grid constraints

Followup to #5090

Marlin/Marlin_main.cpp

@@ -8310,15 +8310,17 @@ void ok_to_send() {
     float ratio_x = x / bilinear_grid_spacing[X_AXIS],
           ratio_y = y / bilinear_grid_spacing[Y_AXIS];
 
-    // Whole unit is the grid box index
-    const int gridx = constrain(floor(ratio_x), 0, ABL_GRID_POINTS_X - 2),
-              gridy = constrain(floor(ratio_y), 0, ABL_GRID_POINTS_Y - 2),
-              nextx = min(gridx + 1, ABL_GRID_POINTS_X - 2),
-              nexty = min(gridy + 1, ABL_GRID_POINTS_Y - 2);
+    // Whole units for the grid line indices. Constrained within bounds.
+    const int gridx = constrain(floor(ratio_x), 0, ABL_GRID_POINTS_X - 1),
+              gridy = constrain(floor(ratio_y), 0, ABL_GRID_POINTS_Y - 1),
+              nextx = min(gridx + 1, ABL_GRID_POINTS_X - 1),
+              nexty = min(gridy + 1, ABL_GRID_POINTS_Y - 1);
 
     // Subtract whole to get the ratio within the grid box
-    ratio_x = constrain(ratio_x - gridx, 0.0, 1.0);
-    ratio_y = constrain(ratio_y - gridy, 0.0, 1.0);
+    ratio_x -= gridx; ratio_y -= gridy;
+
+    // Never less than 0.0. (Over 1.0 is fine due to previous contraints.)
+    NOLESS(ratio_x, 0); NOLESS(ratio_y, 0);
 
     // Z at the box corners
     const float z1 = bed_level_grid[gridx][gridy],  // left-front