PFW-1553 fix regression where SD card file is not closed when stopping paused print

When a print is paused, card.sdprinting is set to false. Instead we can check
if the SD card has been mounted and try to close the file.

The closefile() method will check internally whether or not the file is open.

Firmware/ultralcd.cpp

@@ -5709,7 +5709,7 @@ void print_stop(bool interactive)
     // called by the main loop one iteration later.
     UnconditionalStop();
 
-    if (card.sdprinting) {
+    if (card.mounted) {
         // Reset the sd status
         card.sdprinting = false;
         card.closefile();