MBL: Fix for negative x/y positions

Marlin/Configuration.h

 //#define MESH_BED_LEVELING    // Enable mesh bed leveling.
 
 #if ENABLED(MESH_BED_LEVELING)
-  #define MESH_MIN_X 10
-  #define MESH_MAX_X (X_MAX_POS - (MESH_MIN_X))
-  #define MESH_MIN_Y 10
-  #define MESH_MAX_Y (Y_MAX_POS - (MESH_MIN_Y))
+  #define MESH_INSET 10        // Mesh inset margin on print area
   #define MESH_NUM_X_POINTS 3  // Don't use more than 7 points per axis, implementation limited.
   #define MESH_NUM_Y_POINTS 3
   #define MESH_HOME_SEARCH_Z 4  // Z after Home, bed somewhere below but above 0.0.

Marlin/mesh_bed_leveling.h

 
 #if ENABLED(MESH_BED_LEVELING)
 
+  #define MESH_MIN_X (X_MIN_POS + MESH_INSET)
+  #define MESH_MAX_X (X_MAX_POS - (MESH_INSET))
+  #define MESH_MIN_Y (Y_MIN_POS + MESH_INSET)
+  #define MESH_MAX_Y (Y_MAX_POS - (MESH_INSET))
   #define MESH_X_DIST ((MESH_MAX_X - (MESH_MIN_X))/(MESH_NUM_X_POINTS - 1))
   #define MESH_Y_DIST ((MESH_MAX_Y - (MESH_MIN_Y))/(MESH_NUM_Y_POINTS - 1))