Fix mesh point sign

Most obvious part of #17670

Co-Authored-By: FilippoR <filippo.rossoni@gmail.com>

Marlin/src/lcd/menu/menu_ubl.cpp

                ubl_fillin_amount = 5,
                ubl_height_amount = 1;
 
-static uint8_t n_edit_pts = 1, x_plot = 0, y_plot = 0;
+static uint8_t n_edit_pts = 1;
+static int8_t x_plot = 0, y_plot = 0; // May be negative during move
 
 #if HAS_HEATED_BED
   static int16_t custom_bed_temp = 50;