Fix NAN mesh entries with ABL_BILINEAR_SUBDIVISION (#20143)

Marlin/src/feature/bedlevel/abl/abl.cpp

   #define LINEAR_EXTRAPOLATION(E, I) ((E) * 2 - (I))
   float bed_level_virt_coord(const uint8_t x, const uint8_t y) {
     uint8_t ep = 0, ip = 1;
+    if (x > GRID_MAX_POINTS_X + 1 || y > GRID_MAX_POINTS_Y + 1) {
+      // The requested point requires extrapolating two points beyond the mesh.
+      // These values are only requested for the edges of the mesh, which are always an actual mesh point,
+      // and do not require interpolation. When interpolation is not needed, this "Mesh + 2" point is
+      // cancelled out in bed_level_virt_cmr and does not impact the result. Return 0.0 rather than
+      // making this function more complex by extrapolating two points.
+      return 0.0;
+    }    
     if (!x || x == ABL_TEMP_POINTS_X - 1) {
       if (x) {
         ep = GRID_MAX_POINTS_X - 1;