vt: fix unicode buffer corruption when deleting characters

In the Linux kernel, the following vulnerability has been resolved: vt: fix unicode buffer corruption when deleting characters This is the same issue that was fixed for the VGA text buffer in commit 39cdb68c64d8 ("vt: fix memory overlapping when deleting chars in the buffer"). The cure is also the same i.e. replace memcpy() with memmove() due to the overlaping buffers.