Bug #53296: LONG BLOB value types are not recognized

Fixed the length of system variables to be 2^24 - 1 
as it is documented for MEDIUMBLOB instead of 
2^24.

sql/item_func.cc

@@ -4640,7 +4640,7 @@ void Item_func_get_user_var::fix_length_and_dec()
       decimals=0;
       break;
     case STRING_RESULT:
-      max_length= MAX_BLOB_WIDTH;
+      max_length= MAX_BLOB_WIDTH - 1;
       break;
     case DECIMAL_RESULT:
       max_length= DECIMAL_MAX_STR_LENGTH;