Add parser.is_command(letter, code)

Marlin/src/gcode/gcode.cpp

   * Will still block Gcodes if M511 is disabled, in which case the printer should be unlocked via LCD Menu
   */
   #if ENABLED(PASSWORD_FEATURE)
-    if (password.is_locked && !(parser.command_letter == 'M' && parser.codenum == 511)) {
+    if (password.is_locked && !parser.is_command('M', 511)) {
       SERIAL_ECHO_MSG(STR_PRINTER_LOCKED);
       return;
     }

Marlin/src/gcode/parser.cpp

      *GCodeParser::string_arg,
      *GCodeParser::value_ptr;
 char GCodeParser::command_letter;
-int GCodeParser::codenum;
+uint16_t GCodeParser::codenum;
 
 #if ENABLED(USE_GCODE_SUBCODES)
   uint8_t GCodeParser::subcode;
 
     // Special handling for M32 [P] !/path/to/file.g#
     // The path must be the last parameter
-    if (param == '!' && letter == 'M' && codenum == 32) {
+    if (param == '!' && is_command('M', 32)) {
       string_arg = p;                           // Name starts after '!'
       char * const lb = strchr(p, '#');         // Already seen '#' as SD char (to pause buffering)
       if (lb) *lb = '\0';                       // Safe to mark the end of the filename

Marlin/src/gcode/parser.h

   static char *command_ptr,               // The command, so it can be echoed
               *string_arg,                // string of command line
               command_letter;             // G, M, or T
-  static int codenum;                     // 123
+  static uint16_t codenum;                // 123
   #if ENABLED(USE_GCODE_SUBCODES)
     static uint8_t subcode;               // .1
   #endif
     static bool chain();
   #endif
 
+  // Test whether the parsed command matches the input
+  static inline bool is_command(const char ltr, const uint16_t num) { return command_letter == ltr && codenum == num; }
+
   // The code value pointer was set
   FORCE_INLINE static bool has_value() { return !!value_ptr; }
 

Marlin/src/gcode/queue.cpp

  * keep sensor readings going and watchdog alive.
  */
 inline bool process_line_done(uint8_t &sis, char (&buff)[MAX_CMD_SIZE], int &ind) {
-  sis = PS_NORMAL;
-  buff[ind] = 0;
-  if (ind) { ind = 0; return false; }
-  thermalManager.manage_heater();
-  return true;
+  sis = PS_NORMAL;                    // "Normal" Serial Input State
+  buff[ind] = '\0';                   // Of course, I'm a Terminator.
+  const bool is_empty = (ind == 0);   // An empty line?
+  if (is_empty)
+    thermalManager.manage_heater();   // Keep sensors satisfied
+  else
+    ind = 0;                          // Start a new line
+  return is_empty;                    // Inform the caller
 }
 
 /**