drivers: video: fbcon: fix NULL dereference in fbcon_cursor()

add null-check on function pointer before dereference on ops->cursor

Reported-by:  <syzbot+b67aaae8d3a927f68d20@syzkaller.appspotmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Du Cheng <ducheng2@gmail.com>
Link: https://lore.kernel.org/r/20210312081421.452405-1-ducheng2@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>