Signal an invalid mesh for M420 enable / load

Marlin/src/gcode/bedlevel/M420.cpp

@@ -126,10 +126,14 @@ void GcodeSuite::M420() {
 
   #endif // AUTO_BED_LEVELING_UBL
 
+  const bool seenV = parser.seen('V');
+
   #if HAS_MESH
 
+    if (leveling_is_valid()) {
+
       // Subtract the given value or the mean from all mesh values
-    if (leveling_is_valid() && parser.seen('C')) {
+      if (parser.seen('C')) {
         const float cval = parser.value_float();
         #if ENABLED(AUTO_BED_LEVELING_UBL)
 
@@ -177,10 +181,16 @@ void GcodeSuite::M420() {
         #endif
       }
 
+    }
+    else if (to_enable || seenV) {
+      SERIAL_ERROR_MSG("Invalid mesh.");
+      goto EXIT_M420;
+    }
+
   #endif // HAS_MESH
 
   // V to print the matrix or mesh
-  if (parser.seen('V')) {
+  if (seenV) {
     #if ABL_PLANAR
       planner.bed_level_matrix.debug(PSTR("Bed Level Correction Matrix:"));
     #else
@@ -205,6 +215,8 @@ void GcodeSuite::M420() {
   // Enable leveling if specified, or if previously active
   set_bed_leveling_enabled(to_enable);
 
+  EXIT_M420:
+
   // Error if leveling failed to enable or reenable
   if (to_enable && !planner.leveling_active)
     SERIAL_ERROR_MSG(MSG_ERR_M420_FAILED);