Workaround for skipping large comment blocks

If there are large blocks of comments in the G-code,
the printer may get shot down by its own watchdog.
Watchdog is generally set to 4s and updated only
in manage_heaters (and some other spots in some specific cases).

So far, the code reading the file and feeding it into Marlin
cycles indefinitely until it finds valid G-code lines and fills up the
command queue.

If the block is large enough, the printer cannot read it completely
within those 4s.

A simple workaround - bail out after some consecutive empty/comment
lines to enable other parts of code do their job (especially
manage_heaters).

Tested on MK404, previous FW fails with 600KB of comment lines at the
beginning, this patch survives. The printer even draws some update
on its status screen before starting a real print.

Firmware/cmdqueue.cpp

@@ -573,6 +573,7 @@ void get_command()
   // this character _can_ occur in serial com, due to checksums. however, no checksums are used in SD printing
 
   static bool stop_buffering=false;
+  static uint8_t consecutiveEmptyLines = 0;
   if(buflen==0) stop_buffering=false;
   union {
     struct {
@@ -586,11 +587,12 @@ void get_command()
   while( !card.eof() && !stop_buffering) {
     int16_t n=card.get();
     char serial_char = (char)n;
-    if(serial_char == '\n' ||
-       serial_char == '\r' ||
-       ((serial_char == '#' || serial_char == ':') && comment_mode == false) ||
-       serial_count >= (MAX_CMD_SIZE - 1) || n==-1)
-    {
+    if( serial_char == '\n'
+     || serial_char == '\r'
+     || ((serial_char == '#' || serial_char == ':') && comment_mode == false)
+     || serial_count >= (MAX_CMD_SIZE - 1)
+     || n==-1
+    ){
       if(serial_char=='#')
         stop_buffering=true;
 
@@ -602,6 +604,10 @@ void get_command()
         // so that the lenght of the already read empty lines and comments will be added
         // to the following non-empty line. 
         comment_mode = false;
+        if( ++consecutiveEmptyLines > 250 ){
+            consecutiveEmptyLines = 0;
+            return; // prevent cycling indefinitely - let manage_heaters do their job
+        }
         continue; //if empty line
       }
       // The new command buffer could be updated non-atomically, because it is not yet considered
@@ -638,9 +644,10 @@ void get_command()
 
       comment_mode = false; //for new command
       serial_count = 0; //clear buffer
+      consecutiveEmptyLines = 0; // reached a non-empty line which shall be enqueued
     
       if(card.eof()) break;
-    
+
       // The following line will reserve buffer space if available.
       if (! cmdqueue_could_enqueue_back(MAX_CMD_SIZE-1, true))
           return;