[FREELDR] Regression: Sometimes shows random chars in last two text screen lines

After e56911f I have seen random colorful characters in the last two lines of the freeldr screen.
This occurs only rarely and randomly. For me I get it when compiling MSVC builds locally on my Windows 7 virtual machine and running the resulting ISO in VMware. Also, I have seen the example that The_DarkFire posted in Mattermost
https://chat.reactos.org/reactos/pl/hoqsi5pewbbqpnmjkofisb5j4r.
Here is a patch freeldr-screen-clear-fix.patch that fixes this problem for me. Nothing special is logged because freeldr normally does not output debug log messages without special handling.