Fix M29 for Pronterface SD card upload (#13010)

Marlin/src/gcode/queue.cpp

           gcode_LastN = gcode_N;
         }
         #if ENABLED(SDSUPPORT)
-          else if (card.flag.saving && strcmp(command, "M29") != 0) // No line number with M29 in Pronterface
+          else if (card.flag.saving && command[0] == 'M' && command[1] == '2' && command[2] == '9' && (command[3] == '\0' || command[3] == ' '))
             return gcode_line_error(PSTR(MSG_ERR_NO_CHECKSUM), i);
         #endif
 
 
     if (card.flag.saving) {
       char* command = command_queue[cmd_queue_index_r];
-      if (strstr_P(command, PSTR("M29"))) {
+      if (command[0] == 'M' && command[1] == '2' && command[2] == '9' && (command[3] == '\0' || command[3] == ' ')) {
         // M29 closes the file
         card.closefile();
         SERIAL_ECHOLNPGM(MSG_FILE_SAVED);