EmbeddedPkg: Clear keyboard queue buffer after reading

There is a possibility to retrieve user input keystroke data stored in the
queue buffer via the EFI_SIMPLE_TEXT_INPUT_PROTOCOL pointer. To prevent
exposure of the password string, clear the queue buffer by filling it
with zeros after reading.

Signed-off-by: Nick Wang <nick.wang@insyde.com>

EmbeddedPkg/Drivers/VirtualKeyboardDxe/VirtualKeyboard.c

@@ -348,6 +348,7 @@ Dequeue (
   }
 
   CopyMem (KeyData, &Queue->Buffer[Queue->Front], sizeof (EFI_KEY_DATA));
+  ZeroMem (&Queue->Buffer[Queue->Front], sizeof (EFI_KEY_DATA));
   Queue->Front = (Queue->Front + 1) % QUEUE_MAX_COUNT;
 
   return EFI_SUCCESS;