Merge pull request #4214 from thinkyhead/fix_clear_command_queue

Allow the command queue to be cleared by commands, lcd menus

Marlin/Marlin_main.cpp

 
 static long gcode_N, gcode_LastN, Stopped_gcode_LastN = 0;
 
-static char* current_command, *current_command_args;
-static int cmd_queue_index_r = 0;
-static int cmd_queue_index_w = 0;
-static int commands_in_queue = 0;
 static char command_queue[BUFSIZE][MAX_CMD_SIZE];
+static char* current_command, *current_command_args;
+static uint8_t cmd_queue_index_r = 0,
+               cmd_queue_index_w = 0,
+               commands_in_queue = 0;
 
 #if ENABLED(INCH_MODE_SUPPORT)
   float linear_unit_factor = 1.0;
 
     #endif // SDSUPPORT
 
-    commands_in_queue--;
-    cmd_queue_index_r = (cmd_queue_index_r + 1) % BUFSIZE;
+    // The queue may be reset by a command handler or by code invoked by idle() within a handler
+    if (commands_in_queue) {
+      --commands_in_queue;
+      cmd_queue_index_r = (cmd_queue_index_r + 1) % BUFSIZE;
+    }
   }
   endstops.report_state();
   idle();